ge i os iit 
“ oaths " 
PPA A hee a 


Ente 
Hela 5 


rae : 


SRAEE Delia ate 45 af). S70 Peiren tate e! Senet BGR EE Spates te 

pace sain ies ets TRAE REE IE Petia centr s 

a Ft a : narzate ty te Okt % 

3 rE tT: PEPER AS ry 4 . 

O53 wiptrarere eins Belch Aye ote ete 

, a8 xb Ai fo Mal vets int Wlee dp Aedbgegel =. 
‘ ‘ : . vee 


Uy? tee et 8e 


etl fae oer 
obs Foy 


righ Gdte bie 
wish (Gk a MeCaee Eek SF 


dvtetce % 


<a aaah 

® \ rs y 
i We FRI =e wath they mE ty 

Men wre 

" a i at beh 8 a 


mnbg 


eae 
Mg alee wel 


he sie F Nae 


- 


= j'! rie) 


0 ! 
fie rt 


ono 
oe 


W96TtEC 


REMOTE STORAGE 


7 
/ 


The person charging this material is re- 
sponsible for its return on or before the 
Latest Date stamped below. 

Theft, mutilation and underlining of books 


are reasons for disciplinary action and may 
result in dismissal from the University. 


UNIVERSITY OF ILLINOIS LIBRARY AT URBANA-CHAMPAIGN 


L161—0O-1096 


Digitized by the Internet Archive 
in 2022 with funding from 
University of Illinois Urbana-Champaign 


https://archive.org/details/atomictheoryOOwurt 


THE INTERNATIONAL SCIENTIFIC SERIES. 
VOLUME OX XTkw at 


oat F 
= \ A ot -,. Aen? eee oa - NY a 
Ge g Dy fale, oe ee a ee OT\ - YOUR A 


| dress in the 


INTERNATIONAL SCIENTIFIC SERIES. 


NOW READY. In12mo and bound in cloth. 


. FORMS OF WATER, in Clouds, Rain, Rivers, Ice, and Glaciers. By 
Prof. Joun 'TYNDALL. 50. 
. PHYSICS AND PULITICS; or, Thoughts on the Application of the Prin- 


ciples of “ Natural Selection” and “ Inheritance” 


to Political Society. 
By Water Baeensor. $1.50 


. FOODS. By Epwarp Sirs, M. D., LL. B., F. R.8. $1.75. 

. MIND AND BODY. By ALEXANDER BAL, LL.D. $1.50. 

. THE STUDY OF SOCIOLOGY. By HERBERT Spencer. $1.50. 

. THE NEW CHEMISTRY. By Prof. Josian P. Cooxs, Jr., of Harvard 


Univ rsity. $2.00 


. THE CO RVATION OF ENERGY. By Prof. Batrour STEwARt, 


LL. D., . B.8.. 31.50. 
ANIMAL LOCOMOTION; or, Walking, Swimming, and Fl ng, with a Dis- 
sertation on Aéronauties. By J. B. Perriarew, M.D. Illustrated. $1.75. 


‘ oe IN MENTAL DISEASE. By H. Mavopstey, M. D. 


$1.50 
. THE SCIENCE OF LAW. By Prof. Suetpon Amos. $1.75. 
. ANIMAL MECHANISM. A Treatise on Terrestrial and Aéria] Locomo- 


tion. By EB. J. Marvy. 117 Illustrations. §1.75. 


. THE HISTORY OF THE CONFLICT BETWEEN ge ge eg AND 
17 


SCIENCE. By Joun Wiittsm Draper, M.D., LL.D. $1. 

THE DOCTRINE OF DESCENT, AND DARWINISM. By Prof OscaAR 
Scumript, of Strasburg University. nae 

THE CHEMISTRY OF LIGHT A * PHOTOGRAPHY, By Dr. 
Hermann Vocer. 100 Llustrations. 


. FUNGI; their Nature, Influence, and Usee® ay M. C. Cooxs, LL. D. Ed- 


ited’ by M. J. Berxerey. 109 Illustrations. $1.50. 


. THE LIFE AND GROWTH OF LANGUAGE, By Prof. W. D. Wuir- 


ney, of Yale College. 


$1.50, 
. MONEY AND ‘THE ata OF EXCHANGE. By W. Sranzey 


Jevons, M.A,, F.R.S. $1.7 


.THE NATURE OF LIGHT, with an Account of Physical Optics. By 


Dr. EK. Lommet, Professor in the University of Erlangen. 88 Lllustra- 
tions and a Plate of Spectra in Chromo-lithography. $2.00. 


. ANIMAL PARASITES AND MESSMATES. By M. Van Sead Pro- . 


fessor of the University of Louvain. 83 Illustrations. $1.50 


. ON FERMENTATIONS. By P. Scuti?tzenBEerGer, gee! at the Chemical 


Laboratory at the Sorbonne. 28 Illustrations. 


. THE FIVE SENSES OF MAN. By J. Buxxsteix, O. O. Professor in 


the University of Halle. 19 Illustrations. $1.75. 


. THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Prof. 


ve ye BLASERNA, of the Royal University of Rome. Numerous Wood- 
cuts 


$1.50. 
. STUDIES IN SPECTRUM ANALYSIS. By J. Norman Looxyer. Il- 


lustrations. $2.50. 


. A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By 


Rosert H. Taursron, A.M., C.E., Professor of Mechanical Engineer- 
ing. 163 Llustrations. $2.50. 


‘ EDUCATION AS A SCIENCE. By Avexanper Barn, LL. D., Professor 


of Logic in the University of Aberdeen. $1.75. 


. MODERN CHROMATICS, with Applications to Art and Industry. By 


OapeEn N. Roop, Professor of Physics in Columbia College. 130 original 
Illustrations. $2.00 


. THE HUMAN SPECIES. By A. Dr Quvarreracss, Professor cf An- 


thropology in the Museum of Natural History, Paris. 


. THE C AYPISH : An ‘Introduction to the Study of " Zodlogy. By Prof. 


7. H. Huxiry. 82 Illustrations. $1.75. 


For sale i all booksellers. Any volume sent by mail, post-paid, to any ad- 
nited States, on receipt of price. 


D. APPLETON & CO., Publishers, 1, 3, & & Bond St., New York. 


Ax Gaal sinnabiel 


ince, 


THE | 


we eRSIP} 


Jae 


LIBRA 


x 


B 
Ularies AD WURTZ, \s4 ai Am 


MEMBRE DE L'INSTITUT; DOYEN HONORAIRE DE LA FAOULT. f ’ 
PKOFESSEUR A LA PACULTE DES SCIENCES DE PARIS.™ 


; . Fs = 
TRANSLATED BY 
E. CLEMINSHAW, M.A., F.0.8., F.1.C., 


ASSISTANT-MASTER AT SHERBOKNE SCHOOL, 
? 


we 


NEW YORK: _ 3 
‘D. APPLETON AND COMPANY, es 
1, 8, anv 5 BOND STREET. . ae = S, 
ace...) eee 


CONTENTS. 


BOOK I. 
ATOMS. 
CHAPTER I. 


HISTORICAL INTRODUCTION—RICHTER—DALTON, 


~ Proust: Fixity of ‘Chemical Proportions—Richter : Law of 


Proportionality— Dalton: Hypothesis-of Atoms—Dalton’s 
Notation ° . * . ° ° 


CHAPTER II. 


LAW OF VOLUMES: GAY-LUSSAC—AVOGADRO AND 
AMPERE—BERZELIUS. 


Gay-Lussac: Law of Volumes—Hypothesis of Avogadro—Hy- 
pothesis of Ampére—Interpretations of Berzelius—Berze- 
lius: Corpuscular Theory . : . ; 


> 


CHAPTER III. 


‘ PROUT’S HYPOTHESIS—LAW OF SPECIFIC HEATS— 
ISOMORPHISM, 


Dulong and Petit: Law of Specific Heats—Mitscherlich: 
_ _Isomorphism—Berzelius’s System of Atomic Weights . 


ae BA Ss 


PAGE 


33 


49 


pl CONTENTS. 


CHAPTER IV. 


SYSTEM OF CHEMICAL EQUIVALENTS —EQUIVALENT 
NOTATION, 


Objections to the Principle of Berzelius’s Notation—Discus- 
sion of the Objections brought forward by Gmelin—Incon- 
sistencies of the Equivalent Notation ° 


CHAPTER V. 


PRESENT SYSTEM OF ATOMIC WEIGHTS: GERHARDT 
AND LAURENT—CANNIZZARO. 


Notation of Gerhardt—Ideas of Laurent—Reform of Canniz- 
zaro—Table of Atomic Weights—Law of Volumes—Present 
System of Atomic Weights deduced from the Law of Avo- 
gadro—Apparent Exceptions to the Law of Avogadro 
—Atomic Constitution of the Elements—New Atomic 
Weights in Harmony with the Law of Dulong and Petit — 
Molecular Heats—New Atomic Weights in Harmony with 
the Law of Isomorphism 


CHAPTER VI. 


THE NEW SYSTEM OF ATOMIC WEIGHTS RESPECTS AND 
RENDERS EVIDENT THE ANALOGIES WHICH EXIST 
. BETWEEN BODIES. 


It agrees with Chemical Analogies—Ideas of Dumas—Mende- 
lejeff’s Principle of Classification—New Atomic Weights in 
Harmony with Physical Properties, with Chemical Proper- 
ties e . © e ° ° © ° 


CHAPTER VII. 


ATOMIC AND MOLECULAR VOLUMES, 


Researches of Hermann Kopp—Molecular Volumes of Salts , 


67 


149 


187 


CONTENTS. Vii 


BOOK II. 
ATOMICITY. 


—_—- 


CHAPTER I. 
PAGE 
DEFINITION AND HISTORIC DEVELOPMENT OF THE IDEA OF 
ATOMICITY: . ° : . : ° - 196 
CHAPTER II. 
AFFINITY AND ATOMICITY, TWO DISTINCT PROPERTIES 
OF ATOMS. 
Atomicity a Relative Property of Atoms—Molecular Com- 
pounds ° ° . ; ° e ° e 224 


CHAPTER III, 


CONSTITUTION OF BODIES DEDUCED FROM THE THEORY 
OF ATOMICITY. 


Atomicity applied to the Interpretation of Isomers—Atomicity . 
applied to the Interpretation of Molecular Dissymmetry . 259 


CHAPTER IV. 


HYPOTHESIS UPON THE CONSTITUTION OF MATTER. 


Conclusion, A - = . 16 . 3805 


Vili CONTENTS. 


APPENDIX. 


NOTE I. 


WATER OF CRYSTALLISATION . 3 - a ++ 33a 


NOTE II. 


THE CONSTITUTION OF DOUBLE SALTS - e - $834 


NOTE IIL. 


THE ISOMERISM OF THE AMYL ALCOHOLS . ° « vaG 


NOTE IV. 


THE ACTION OF HEAT UPON GASES, ; ‘ ; >; 888 


PLATE. 


THE RELATIONS BETWEEN THE ATOMIC WEIGHTS OF THE 
ELEMENTS AND THEIR PHYSICAL PROPERTIES, AFTER 
LOTHAR MEYER - i - . < * = at end 


THE ATOMIC THEORY. 


—_— oO 


PG Orr. 
ATOMS. 


CHAPTER I. 


HISTORICAL INTRODUCTION—-RICHTER— DALTON. 


Tue hypothesis of atoms, put forward by the Greek 
philosophers, and revived in modern times by great 
thinkers, acquired a definite form at the begin- 
ning of this century. John Dalton was the first to 
apply it to the interpretation of the laws which he and 
Richter recognised as governing chemical combinations. 
Confirmed by the great discoveries of Gay-Lussac, 
Mitscherlich, Dulong and Petit, the hypothesis has 
assumed a definite form, connecting many various facts 
of a chemical and physical nature. Fundamentally it 
consists of modern ideas upon the constitution of 
matter. ~ 

In common with correct ideas, it has grown with 
time, and nothing has as yet happened to stop its pro- 


2 THE ATOMIC THEORY. 


gress; but, in common with all fruitful ideas, it has been 
an instrument of progress even in the hands of its 
detractors. The latter are now few, and the hypothesis 
seems to make a firm stand against the regular opposi- 
tion of some and the subtle attacks of others. In these 
pages we propose to discuss both its historical evolution 
and its present form, and we shall thus show the influ- 
ence it has exercised upon the progress of science since 
the beginning of the century. 

Dalton revived the hypothesis of atoms to explain 
the fact that in chemical combinations elements unite 
in fixed proportions, and in certain cases in multiple 
proportions. He admitted that these proportions repre- 
sent the relative weights of indivisible particles of 
the bodies, which particles are brought into contact 
and grouped by the fact of combination. This led to 
the consideration of atomic weights, and the idea of 
representing the composition of bodies by symbols which 
indicate both the nature and the number of these 
particles and the proportion of the elements entering 
into combination. We have here two things which 
must not be confounded— facts and an hypothesis. We 
shall retain the hypothesis as long as it gives a faithful 
interpretation of facts, and enables us to group them, 
to connect them together, and to anticipate fresh ones 
—as long, in fact, as it proves fertile. An hypothesis 
thus formed rises to the rank of a theory. We shall 
endeavour to show, in demonstrating its origin, progress, 
and results, that this is the case with Dalton’s concep- 
tion. 


DEFINITE CHEMICAL PROPORTIONS. 3 


Ps 


Simple bodies combine in definite proportions. This 
is one of the most firmly established truths of natural 
philosophy. It includes the two following facts :— 
Firstly, the relative weight of combining bodies is always 
fixed in every combination; secondly, the numbers which 
express these relations are interproportional for all 
kinds of combinations. We must clearly understand 
the meaning of these propositions. 

Two simple bodies unite so as to form a given com- 
pound. As long as the compound lasts the relative 
weights of the two elements will remain perfectly con- 
stant, whether the quantities acting upon each other 
have been great or small; the smallest particles, as 
well as the whole mass, will contain strictly proportional 
weights of these elements, which no physical circum- 
stances, such as pressure or temperature, can modify. 
This is true for all kinds of combinations, the most 
simple as well as the most complicated. This fixity of 
the proportion in which bodies combine was acknow- 
ledged and admitted as a truth more than a century 
ago by some eminent chemists, and by all in the year 
1806. Bergman was conscious of the truth, even if not 
logically convinced of it; in fact, the numerous quan- 
titative analyses for which we are indebted to him 
would have been aimless or useless if he had been under 
the impression that the compounds he was analysing 
were formed in chance proportions. Lavoisier demon- 
strated in the clearest manner the fact of the constancy 
of the relations in which bodies combine. In every 


4 THE ATOMIC THEORY. 


oxide, in every acid, he said, the relation of oxygen to 
the metal is constant; and this relation should be 
exactly determined for every oxygen compound. He 
admits, moreover, that the difference between the acids 
of sulphur and the oxygen compounds of nitrogen is 
due to the power possessed by these simple bodies of 
uniting with oxygen in several proportions, each degree 
of oxidation corresponding to a fixed and constant rela- 
tion between the weights of the two elements. The 
law of fixity was thus distinctly admitted and clearly 
stated by Lavoisier ; one step more, and he would have 
discovered the law of multiple proportions. He did 
not, however, make this decisive step. Even as regards 
the fixity of several proportions, though he was himself 
convinced of the fact, he was not successful in making 
it universally accepted. In the month of July 1799 
his pupil Berthollet read at the Egyptian Institute, 
which was sitting at Cairo, a memoir entitled ‘Researches 
upon the Laws of Affinity.’ He there for the first time 
brought forward profound ideas upon the influence 
exercised by the physical condition, the cohesion, solu- 
bility, insolubility, and volatility of bodies upon the 
affinity and progress of chemical decompositions. With- 
out denying the fixity of the composition of certain 
compounds, he attributed this fact to the chance influ- 
ence of these physical conditions, which in some cases 
were constant, and would not allow that it partook of 
the character of a general law. 

It is true, he said, that in sulphate of baryta the 
relation between the sulphuric acid and the baryta is 
constant, simply because the acid and base must 


PROUST—FIXITY OF PROPORTIONS. 5 


unite in this precise and fixed proportion to form a salt 
of absolute insolubility. Thus, here as in many other 
eases, constancy of composition is dependent upon a 
physical property, cohesion—in other words, the insolu- 
bility of the sulphate of baryta. But the rule is as 
follows: Chemical combinations take place in propor- 
tions which may vary within certain limits. 

A salt formed by a soluble acid and a slightly solu- 
ble or insoluble base may be precipitated in an insoluble 
form, unvarying in composition, when the proportion of 
the base is exactly such as to cause the precipitation of 
the salt of this composition ; but if the proportion of 
the base is increased the salt will still be precipitated, 
but its composition will be different, for it now consists 
of greater quantities of base for the same quantity of 
acid. 

A metal, such as mercury, dissolved in nitric acid, 
will unite, in the process of oxidation, with quantities 
of oxygen varying between a maximum and a minimum. 
We canrfot, therefore, maintain with Lavoisier that 
when a salt is formed by the action of an acid upon a 
metal, there is a constant relation between the quantity 
of the metal and the quantity of oxygen which the 
former takes from the acid in the process of oxidation. 

These propositions of Berthollet were first opposed 
and successfully refuted by S. L. Proust. Having 
remarked, in 1799, that upon dissolving native carbonate 
of copper in an acid, and then precipitating the solution 
' by an alkaline carbonate, he obtained a quantity of car- 
bonate of copper equal to that of the native carbonate 
which had been dissolved, Proust drew from this fact 


6 THE ATOMIC THEORY. 


the conclusion that the composition of carbonate of 
copper is fixed and invariable, whether the salt has been 
formed in the depths of the earth or artificially by a 
chemical process. His subsequent researches enabled 
him to generalise this conclusion; and in speaking of 
these researches we must specially quote those upon the 
composition of the two oxides of tin, the sulphides of 
iron, and sulphide of antimony. In all these com- 
pounds the relation in weight between the two elements 
is constant ; and if two simple bodies, by combining in 
different proportions, are able to form several com- 
pounds, as is the case with tin and oxygen, iron and 
sulphur, it is evident that in every degree of combina- 
tion the relation in question is invariable. 

Proust brought forward these facts, which he had 
discovered in opposition to those upon which Berthollet 
took his stand, and showed that the latter allowed a 
different interpretation. Metallic solutions, where the 
metal enters into combination with variable quantities 
of oxygen ;-salts, which, when precipitated, may contain 
variable quantities of bases ; or oxides of tin and lead, 
which have been obtained by the calcination of metals 
in contact with air, and which have fixed variable 
quantities of oxygen—in no case consist of, or constitute, 
definite chemical compounds, but are mixtures, in dif- 
ferent proportions, of several compounds, all of which 
possess a fixed composition. The fixity of composition, 
indeed, seemed to Proust an essential attribute of com- 
binations, a great law of nature—the pondus nature, 
justly recognised by Stahl. 

This discussion, which is one of the most memorable 


FIXITY OF PROPORTIONS. 7 


of which science possesses a record, lasted from 1799 till 
1806, and was maintained on both sides with a power 
of reasoning and a respect for truth and propriety 
which have never been surpassed. The fullest deve- 
lopment of Berthollet’s views appeared in his celebrated 
work entitled ‘ Essai dune Statique chimique,’ which 
was published in 1803. The great idea developed in 
this book is that chemical affinity and astronomical 
attraction are different manifestations of an identical 
property of matter, which led the author to regard not 
only the energy of affinities as producing chemical 
reactions, but also the influence of the masses. 

In a great number of reactions this influence does 
undoubtedly govern the progress of decomposition or 
combination ; it augments or diminishes the proportion 
of compounds which are formed or destroyed in a reac- 
tion, but it does not govern the proportions in which 
the elements unite in these compounds. On this latter 
point Berthollet held a different opinion; he main- 
tained that mass does exercise an influence upon the 
combining proportions of two bodies when no physical 
condition is present to determine the separation of a 
compound in fixed proportions. Thus, when an acid 
acts upon a base in such a manner as to produce a solu- 
ble salt, the point of neutrality undoubtedly corresponds 
to fixed proportions-of combined acid and base; but if 
an excess of one or other of these elements be added, 
it also will enter into combination, and, moreover, in 
variable proportions, till a physical property—cohesion, 
for example—determines the separation of a compound 
of fixed proportions. In a great number of chemical 


8 THE ATOMIC THEORY. 


combinations, therefore, this fixity in the proportions 
of elements may be observed; but, in the opinion of 
Berthollet, they are exceptional cases, to which it 
would be wrong to ascribe the dignity of a general law. 

Proust, on the contrary, maintained the generality of 
this law. If it is impossible, he says, to make an ounce 
of nitric acid, an oxide, a sulphide, or a drop of water 
in other proportions than in those which nature, from 
all eternity, has assigned to these compounds, we must 
acknowledge that for chemical combinations there is a 
sort of ‘ balance,’ which is subject to the immutable laws 
of nature, and which, even in our laboratories, deter- 
mines the relation of the elements in these compounds. 
The latter are of several orders. The most simple are 
generally formed of two elements—at most of three, very 
rarely of four. But these compounds of a simple order 
may combine with each other, so as to form more com- 
plex compounds; in other cases they are merely mixed 
together. In these mixtures the proportion of the 
elements is naturally subject to variation; in all che- 
mical combinations properly so called it is, on the 
contrary, fixed. 

The opinion of Proust was well founded; it won 
the day, in spite of the opposition of his powerful 
antagonist ; and we cannot too much admire the per- 
severing energy and discernment displayed by the 
chemist of Angers in this contest, when he took one 
by one the arguments of Berthollet, and opposed to 
the facts collected and arranged by the latter in support 
of his theory fresh facts and fresh analyses of his own, 
which, it must be confessed, were not always models of 


FIXITY OF PROPORTIONS. 9 


accuracy. The superior intelligence, however, of an 
accurate and lofty mind saved him from error in the 
discussion of results, and made up for the insufficiency 
of the methods of that time. 

This great truth of the fixity of chemical propor- 
tions was, then, definitely established in the year 
1806. But the discussions between Berthollet and 
_ Proust, which agitated the scientific world during the 
first years of this century, only gave an incomplete idea 
of it, for they dealt solely with the composition of each 
compound taken individually. The question as to 
whether sulphide of antimony was a constant com- 
pound, and whether this was also the case with the 
sulphides of iron, the oxides of tin and cobalt, was 
answered in the affirmative by Proust, in the negative 
by Berthollet. It is now definitely decided in the 
affirmative. We must not, however, forget that Proust 
and Berthollet only attacked the question from one 
side, for there is another. It is true that this sulphide 
of antimony, these sulphides of iron, and, in fact, that 
all sulphides present a fixed composition; and, again, 
it is equally true that in every metallic oxide the 
metal and the oxygen unite in invariable proportions. 
But this is not all. Analysis shows, further, that the 
relations between quantities of different metals uniting 
with a fixed weight of sulphur are the same as those 
between different metals uniting with a fixed weight of 
oxygen. Independently, therefore, of the fact of fixity, 
there is the further fact of the proportionality of 
the combining quantities or weights of bodies; and 
the case in question is not an exceptional one, but 


10 THE ATOMIC THEORY. 


belongs to a whole order of. similar facts—is, in short, 
a law. 

We have, in demonstrating this law of propor- 
tionality, employed as examples the very compounds 
which enabled Proust to establish the law of fixity. It 
may, however, be demonstrated under a more general 
and striking form. 

A is a certain weight of a simple body. 

B is a certain weight of another simple body, which 
is exactly sufficient to form with a the combination a B. 


. A . 
The relation — is constant. 
B 


C is a certain weight of a third simple body, exactly 
sufficient to form with a the combination ac. The re- 


ay Oe 
lation — is constant. 
C 


D is a certain weight of a fourth simple body, exactly 
sufficient to form with a the combination ap. The 


. A J 
relation — is constant. 
D 


This is Proust’s law. 

Let us now take the second body B, and form com- 
binations between this body and the third c and the 
fourth p. Experience shows us that the quantities c 
and Dp which combine with a will also combine with 3B 
—in other words, that the weights of the bodies 3, c, p, 
which formed definite compounds with A, are unchanged 
when they combine with each other. From the fact of 
the existence of compounds AB, AC, AD, We may assume 
the existence of compounds BC, BD, CD, in which the 
quantities A,B, C, D, are constant. In short, there 


LAW OF PROPORTIONALITY—RICHTER. ¥y 


exists between all compound bodies formed by the union 
of two elements such a definite relation of composition 
that we have only to determine the proportions in 
which the most widely differing elements unite with 
one of their number, and we shall also have determined 
the proportions in which they combine with each 


other. 
This is the law of proportionality, discovered by 


Richter, who lived at Berlin towards the close of the 


last century. 
For many years another German chemist—C. F. 


Wenzel—was considered the author of this great dis- 
covery. It was attributed to him by Berzelius.' M. 
Dumas also claims it for him,? and all chemical treatises 


1 The following are the terms in which Berzelius claimed for 
Wenzel the discovery of the proportionality of quantities of acids 
and bases which exactly saturate each other:—‘ He published the 
result of these experiments in a memoir entitled Lehre von den 
Verwandtschaften, or the Theory of Affinities, at Dresden in 1777, 
and proved, by singularly accurate analyses, that this phenomenon 
(the preservation of neutrality after the mutual decomposition 
of two neutral salts) was due to the fact that the quantities of 
alkalies and earths which saturate a given quantity of the sameacid 
are the same for all acids; so that if we decompose, for example, 
calcium nitrate by potassium sulphate, the potassium nitrate and 
the calcium sulphate obtained will preserve their neutrality, 
because the quantity of potash which saturates a given quantity 
of nitric acid is to the quantity of lime which saturates the same 
quantity of nitric acid as the potash is to the lime which neutralises 
a given quantity of sulphuric acid.’— Yraité de Chimie, French 
edition, 1831, t. iv. p. 524. 

2 Chemical Philosophy, p. 200. The error concerning the part 
attributed to Wenzel in the discovery of the law of proportionality 
has been corrected by several scientific writers—first by Hess 
(Journal fiir praktische Chemie, t. xxiv. p. 420); then by Schweigger, 
in the work‘ entitled Ueber stéchiometrische Rethen im Sinne 


12 THE ATOMIC THEORY. 


fifty years ago quoted him as the precursor of Richter. 
He was rather the rival of Bergman and Kirwan. The 
analyses of neutral salts which he published were accu- 
rate ; but he nowhere mentions the fact of the reserva- | 
tion of neutrality after the double decomposition of | 
the two neutral salts; he admits, on the contrary, that 
in the phenomenon in question the quantity of the two | 
neutral salts which react upon each other being cal- 
culated after their known composition, a certain excess _ 
of one of the elements may remain after the decom- 
position has taken place. This opinion is contrary to 
facts, and must necessarily have rendered it impossible 
for the author to discover the law of proportionality. — 
This law was demonstrated a few years later by a much — 
less experienced chemist than Wenzel, who was obscure 
and diffusive in his productions, but endowed with 
singular penetration and rare perseverance. 


ee 


a 


iT: 


J. D. Richter was preoccupied with the idea of 
applying mathematics to chemistry, and particularly to 
that of discovering numerical relations between combin- — 
ing bodies. His efforts in this direction did not meet 
with success; for, though he was the first to recognise 
and demonstrate the law of proportionality between 


_, the quantities of bases uniting with a given weight of 


Rai pts 


' base, and between the quantities of acids uniting with a 
given weight of base—a most important and well-esta- 
blished fact—he fell into error in trying to show that these 


Richter’s, Halle, 1853 ; lastly by R. A. Smith (Memoir of J. Dalton, 
and History of the Atomic Theory up to His Time, London, 1856). 


LAW OF PROPORTIONALITY—RICHTER. 13 


quantities form numerical series, the terms of which bear 
to each othera simple ratio.! But'we need not pay much 
attention to this point. Let us rather gather from the 
work of Richter the great truths and fundamental dis- 
coveries which demand the grateful recognition of pos- 
terity, all the more strongly from the fact that they were 
neglected and almost ignored by his contemporaries. 
Richter founded his researches upon the then well- 
known fact of the permanence of neutrality in the 
double decomposition of two neutral salts. Richter 
found and clearly demonstrated the required explana- 
tion of this fact. In the first volume, published in 


1 Richter tried to show that the quantities of bases which saturate 
a given weight of acid represent the terms of an arithmetical pro- 
gression, and that the quantities of acids which combine with a 
given weight of base form the terms of a geometrical progression. 
Thus, for example, he found that 1,000 parts of hydrochloric acid 
are saturated by-734 of alumina, 858 of magnesia, 1,107 of lime, 
and by 3,099 of baryta. These numbers form the terms of a series 
a, a+b, a+3b, a+19b, in which a@=734 and 0=1245. Having 
afterwards discovered the saturating capacity of strontia for hydro- 
chloric acid, he found that this base would occupy the place @+116 
in the preceding series, a result which he soon corrected to a+ 9b. 

A different but very simple relation exists, in his opinion, 
between the quantities of acids which saturate a given quantity of 
base. Thus the quantities of fluoric (hydrofluoric) acid 696-4, 
muriatic acid 1160-0, sulphuric acid 1630:0, and nitric acid 2290-4, 
which saturate 1,000 of magnesia, form the first, third, fourth, and 
fifth terms of geometrical progression—e, cd, cd*, ed*®, cd’—the first 
term of which c is = 696°4, and d=1°1854, Again, the quantities of 
carbonic, sebacic, oxalic, formic, succinic, acetic, citric, and tartaric 
acid necessary to neutralise a given base increase according to a 


geometrical progression a, ab, ab*, ab’. Metallic acids, on the con- 


trary, are subject to another law: the quantities of tungstic, 
chromic, arsenic, and molybdic acid which saturate a given weight 
of base constitute the terms of an arithmetic progression. 


14 THE ATOMIC THEORY. 


1792, of his ‘ Elements of Stoichiometry’! he expresses 
himself as follows :—Let a and B represent the weights 
or masses of two neutral compounds (salts) which 
exactly decompose each other; the new bodies will 
remain neutral: let a represent the mass of an element 
in A, and b that of an element in B; the masses of the 
two elements in a will be a, A—a, and in B will be 
b, B—b. Before decomposition the ratio of the masses 
(weights) of the neutral compounds A and B will 
a b 


22s and sae 
A—a B—b 


After decomposition the masses of the elements in the 


All these propositions are founded upon inaccurate data, a fact 
which doubtless did not escape the notice of some of Richter’s con- 
temporaries, and contributed to throw discredit upon his labours. 
He himself sometimes saw the necessity of correcting some of these 
errors; but though he gave up a few details, he still held to the 
numerical laws demonstrated above—the new figures always adapted 
themselves to it. Thus in 1797 soda changes its place in the series 
of bases neutralising a given weight of sulphuric acid. Richter now 
finds that 1,000 parts of this acid are saturated by 672°1 of volatile 
alkali (instead of 638), by 858°6 of soda (instead of 1,218), and by 
1604:6 of potash (instead of 1,606). These numbers increase as the 
terms a, a+b, a+ 5b, while the original numbers formed the terms 
of a series a, 4+ 3b, a+ 5d. ; 


These are great imperfections in the work of Richter; but, 


though we cannot but regret that his memory should be charged 
with them, they must not cause us to forget the great truths which 
he had the honour of discovering. — 

1 Anfangsgriinde der Stichiometrie, oder Messkunst chemischer 
Elemente. Pa 

2 We have reversed these fractions, which the author wrote— ~ 


A—@ B—b 
ee d . 
an b 


iin. 


4 « 
a. 
4 2 
—— 


LAW OF PROPORTIONALITY—RICHTER. 15 


new products will be a, B—b and b, a—a, and the 
ratio of these masses will be 


a 
tee Atl 


B—b A—G@ 


If, then, the ratio of the masses (elements) is recog- 
nised in the original compounds, the same ratio must 
be acknowledged in the new compounds. 

Richter drew up, in 1793, a table which he termed 
series of masses—the quantities of analogous elements 
(acids or bases) which combine with a given weight of 
another element. In another part of the work which 
we have just quoted he definitely states the following 
proposition :—The different quantities of bases which 
form neutral salts with 1,000 parts of anhydrous 
muriatic acid also form neutral salts with a given 
weight (1,394 parts) of anhydrous sulphuric acid. It 
follows, from the formula given above, that if we take 
a weight a of a muriate (chloride) containing 1,000 
parts of acid and a weight a—1000 of base, and a weight 
B of a sulphate containing 1,394 of sulphuric acid, and 
B—1394 of a second base, this quantity of the latter 
base will exactly neutralise 1,000 parts of muriatic 
acid, while the quantity a—1000 of the first base will 
exactly neutralise 1,394 parts of sulphuric acid. 

If, therefore, we mix the two original salts, the 
neutral muriate and sulphate, we shall obtain from the 
double decomposition a new sulphate and a new muriate, 
which again will be neutral. Richter thus explains the 
fact of the permanence of neutrality when two neutral 
salts exchange their bases and acids. He at this time 


16 THE ATOMIC THEORY. 


(1793), and in the same work, gave the first ‘series of 
masses’ for the alkaline bases and for the earths—that is 
to say, the equivalent quantities of bases which saturate 
a given weight (1,000 parts) of sulphuric, hydrochloric, 
and nitric acids. 

The following is the series :— 


Sulphuric Acid | Muriatice Acid Nitric Acid 


Potash. . ] : 1,606 2,239 1,143 
Soda . ‘ 4 : 1,218 1,699 867 
Volatile alkali . ‘ 638 889 453 
Baryta . , ; : 2,224 3,099 1,581 
ene ee BPs ine 796 1,107 565 
Magnesia . ; ; 616 858 438 
Alumina : i : 526 734 374 


Although these figures are far from correct, they 
allow the deduction of the law of proportionality, with 
which the name of Richter is justly connected. He 
afterwards completed and corrected them. Having 
ascertained the quantities of lime and potash which 
neutralise 1,000 parts of fluoric (hydrofluoric) acid, he 
proved that these quantities are very nearly propor- 
tional to those which neutralise 1,000 parts of muriatic 
acid. On this point he affirms that ‘the masses of 
alkalies or alkaline earths, when they maintain neutrality 
-with a given mass of either of the three other volatile 
acids,' will always bear to each other the same ratio.’ 
The idea is correct, though the form of expression is 
not happy. Richter, indeed, generally failed in the 
latter respect. Thus he endeavours to generalise the 


1 Sulphuric, muriatic, and nitric acids. 


RICHTER—LAW OF PROPORTIONALITY. 17 


law he has discovered by terming the substance (the 
acid, for example) which enters into combination with 
a series of analogous substances (bases) the determining 
element, and the latter the elements determined. ° 

Let P represent the mass of a determining element, 
the masses of ‘its’ elements determined being a, ), ¢, 
d, e, &c.; Q the mass of another determining element, 
a, B; y, 5, ¢, &c., being the masses of ‘its’ elements 
determined; so that a@ and a, b and £, ¢ and y, d and 
6, and e and « shall represent the same elements; and, 
further, that p+a@ and q+, P+) and Q+y¥, P+e and 
Q+a, &c., are decomposed by double affinity, so that 
the new products will remain neutral. We shall observe 
that the masses a, b, c, d, e, &c., bear to each other the 
same ratio as the masses a, 8, y, 6, ¢, &c. Such is the 
discovery of Richter as he himself published it in 1795 
in the fourth part of his ‘ Mittheilungen uber die 
neueren Gegenstiinde der Chemie.’ 

This is not all. We owe to his penetration another 
important discovery which is closely connected with the 
one we have just mentioned. 

We shall now direct our attention to metallic salts 
properly so called. When two of these salts are de- 
composed by double affinity—that is to say, when they 
exchange their acids and bases—the metal of the one 
finds in the other exactly the quantity of oxygen neces- 
sary to keep it dissolved in the acid; in other words, 
the quantities of different metals necessary for the 
formation of neutral salts absorb the same quantity of 
oxygen when they dissolve in a given weight of acid. 
This proposition, which is, moreover, very accurate, 

2 


18 THE ATOMIC THEORY. 


assumed a clearer form when Lavoisier some time 
afterwards worded it thus :—The different quantities of 
oxides which combine with a given weight of acid con- 
tain the same quantity of oxygen. Richter followed 
up these investigations with great success. He admits 
that the ratios in which oxygen combines with other 
bodies, particularly metals, are perfectly fixed, and that 
the quantity of oxygen fixed by a metal during solution 
in an acid is not always the same as that which it 
absorbs when heated in contact with air. He is thus 
led to distinguish several degrees of oxidation, notably 
in the case.of iron and mercury. ‘The latter forms two 
oxides capable of producing salts. Each of these salts 
presents a perfectly fixed composition, and passes with- 
out alteration of composition by double exchange from 
one salt to another. These researches date from the 
close of the last century, and it seems as if, from the 
manner in which they were conceived and expressed, 
the influence of Lavoisier had made itself felt, unknown 
to the author and in spite of his opposition to the 
doctrines of the reformer. The very fact of this oppo- 
sition seems, in a great measure, to have been the cause 
of the discredit thrown upon the labours of Richter ; 
his time was not yet come; other topics created more 
interest; and in Germany, as also in France and 
England, men’s minds were engrossed by the influx of 
new ideas. | 

There is some difficulty in harmonising the signifi- 
cation and even the publication of Richter’s great dis- 
coveries with the phlogistic theories which he main- 
tained, and which apparently influenced his observations. 


—- 


RICHTER—LAW OF PROPORTIONALITY. 19 


Strictly speaking, we can understand that he could have 
regarded acids as undecomposable bodies, for he only 
considered their relative weights, which are independent 
of their constitution. But when we turn to his opinions 
upon the nature of oxides, upon the fixity of their com- 
position, upon the equality of the weights of oxygen 
absorbed by metals when dissolved in equivalent quan- 
tities of acid, how can we reconcile these correct and 
simple notions with the erroneous conception of phlo- 
giston? It must be confessed that Richter adapted all 
this to his theory. He held that the metallic calces or 
oxides were formed by the combination of metals with 
oxygen, causing a loss of imponderable phlogiston and 
most curious contortion of the phlogistic theory. Had 
he but said heat instead of phlogiston, he would have 
been quite right. We may, therefore, absolve Richter 
on this head ; but his contemporaries were more severe, 
and he himself confesses that in 1799 he was declared 
by the partisans of antiphlogistic doctrines to have 
taken leave of his senses, 

The profound but perplexed author of the great 
discovery in question—the proportionality which exists 
between the weights of elements in chemical combina- 
tions—was fortunate in having an intelligent and 
ingenious commentator. G. E. Fischer published in 
1802 a German translation of Berthollet’s ‘ Researches 
upon Affinity,’ and further endeavoured in this work to 
explain and simplify the deductions which Richter made 
from the fact of the permanence of neutrality after the 
decomposition of two neutral salts. He succeeded, and 
simplified the demonstration of the law of proportionality 


20 THE ATOMIC THEORY. 


in the following manner :—Richter had given a series of 
neutralisation for each acid and each base; he had 
determined the quantities of bases which saturate 1,000 
parts of sulphuric acid, 1,000 parts of nitric acid, and 
1,000 parts of hydrochloric acid; and then, again, he 
had indicated the quantities of acids which would 
saturate 1,000 parts of each base. Though admitting 
that the quantities of acids and the quantities of bases 
composing these series are proportional, he uselessly 
multiplied the number of the latter. Fischer saw that 
they might be reduced to one by giving the ratio which 
the quantities of acids and bases contained in the series 
bear to one number, 1,000 parts of sulphuric acid. In 
fact, he drew up the first table of chemical equivalents 
as follows :— 


Bases. Acids. 

Alumina. : pips Fluoric acid. o 427 
Magnesia ‘ . 415 Carbonic ,,; « pr tact 
Ammonia : . 572 Sebacic mh? Se - 7; UG 
Lime ; : Athy © Muariatic’. 5;7°3 e TiS 
Soda 4 : fate tits, Oxalic wae a FLO8 
Strontia . ; . 1,829 Phosphoric,, . uo Long 
Potash . A . 1,605 Formic Seas oat ists) 
Baryta . : . 2,222 Sulphuric ,, . . 1,000 
Succinic  ,, . . 1,209 

Nitric ak . 1,405 

| Acetic vs hee . 1,480 

Citric a ate . 1,583 

Tartare? 05,°% . 1,694 


The figures in these two columns represent equiva- 
lent quantities of acids and bases. To neutralise a 
given base of the first series with a given acid of the 
second series, we must take of that base and that acid 
the quantity indicated by the accompanying figures. 


ane 
ri 


RICHTER—LAW OF PROPORTIONALITY. 21 


The ratios of neutrality between the bases and acids are 
expressed by these numbers, and the table demonstrates 
in a striking and convenient form the coon a 
of a large number of neutral salts. 

The foregoing table forms part of a note which was 
inserted by Fischer in Berthollet’s ‘ Chemical Statics.’ ! 
It is to his translator that the latter owed his 
acquaintance with the researches of Richter. He had 
treated of the same subject in a chapter of the ‘Statics’ 
entitled * Acidity and Alkalinity,’ and had mentioned 
the opinion of Guyton de Morveau upon the inference 
which may be drawn from the permanence of neutrality 
after the decomposition of certain neutral salts, so as 
to calculate beforehand or control the composition of the 
salts produced. Both chemists, however, acknowledged 
that Richter had anticipated them in this direction. 
Berthollet expresses himself on this point as follows :— 
‘The preceding observations appear to me necessarily to 
lead to the conclusion that in my researches I have only 
hinted at the laws of affinity, but that Richter has 
positively established the fact that the different acids 
follow proportions corresponding with the different 
alkaline bases in order to produce neutrality. This fact © 
may be of the greatest utility in verifying the experi- 
ments which have been made upon the proportions of 


. the elements of salts, and even to determine those 


which have not yet been decided by experiment, and so 
furnish the surest and easiest method of accomplishing 
this object, so important to chemistry.’ 
Thus Berthollet admitted the law of proportionality, 
1 Vol. i. p. 134, 1802. 


22 THE ATOMIC THEORY. 


discovered by Richter, though at the same time he 
questioned the fixity of certain chemical combinations. 
He considers it possible for neutral salts, precipitated in 
an insoluble state or separated as crystals from their 
solutions, to exist in physical conditions compatible, 
according to him, with a fixed composition. 

As we remarked above, we owe to Richter another 
important discovery. He observed that the quantities 
of different metals which dissolve in a given weight of 
acids also combine with the same weight of oxygen. 
This discovery met with no recognition, and was made 
afresh by Gay-Lussac in 1808. It was the same with 
the following fact, which Richter established: that cer- 
tain metals, such as iron and mercury, have the power of 
combining with oxygen in several proportions, so as to 
form two degrees of oxidation. Proust rediscovered 
this fact, and laid great stress upon it in his discussion 
with Berthollet, but he failed to observe that the quan- 
tities of oxygen contained in the different oxides of a 
given metal increase in a very simple ratio.! 

We find, therefore, that at the close of the last and 
the commencement of the present century a number of 
definite facts were discovered concerning the composi- 
tion of salts and chemical compounds in general, but 
that these facts were isolated and without connection. 
Their deep signification escaped the observation of 

1 Proust admitted that 1,000 parts of copper combine with 172 
to 18 parts of oxygen to form the first or sub-oxide of copper, and 
with 25 parts of oxygen to form the second or black oxide. The 
correct numbers are 12°6 and 25:2. Had the analyses of the two 


oxides been more correct, Proust might have recognised the law of 
multiple proportions, 


DALTON—HYPOTHESIS OF ATOMS. 23 


chemists, and the theoretical link which unites them 
was entirely unknown. It was reserved for an English 


_ chemist to complete them by a discovery of the first 


order and to arrange them by an hypothesis both simple 


and fruitful. 


Ill. 


In 1802 John Dalton, at that time a professor in 
Manchester, was investigating the action of air upon bin- 
oxide of nitrogen in the presence of water. He observed 
that the oxygen contained in 100 volumes of air 
united with either 36 or 72 volumes of binoxide of 
nitrogen, leaving a residue of pure nitrogen gas above 
the water. He concluded from this fact that oxygen 
combined with a certain quantity of binoxide of nitro- 
gen or with double that quantity, but not with any 
intermediary quantities, nitric acid being formed in the 
first instance, nitrous acid in the second. In this - 
observation we have the germ of the law of multiple 
proportions, although it was not as yet formally stated 
in the memoir in question.' It was announced at the 
same time as the atomic theory, by which it is theoreti- 
cally explained, in a communication by Dalton to 
Thomson in August 1804. He was then studying the 
composition of marsh gas, and observed that for the 
same quantity of carbon this gas contains a quantity 
of hydrogen exactly double that which is contained 


1 Memoirs of the Literary and Philosophical Society of Man- 
chester, Vol. Vv. p. 535. 


24 THE ATOMIC THEORY. 


in bicarburetted hydrogen. We learn further from 
Thomson ! that the foundations of Dalton’s theory were 
derived from his researches into the composition of 
combinations of oxygen and nitrogen, and that, in fact, 
the observation mentioned above upon the absorption of 
oxygen by binoxide of nitrogen first gave him an in- 
sight into the composition of those combinations. Be- 
yond this it is difficult to affix any precise date to the 
discoveries of Dalton, or at least to trace the logical 
sequence and successive evolution of his ideas, and to 
separate the origin of the law of multiple proportions 
from the origin of the conception of the atomic theory. 

In fact, in a memoir upon the absorption of gases 
by water, read in October 1803 before the Literary and 
Philosophical Society of Manchester, Dalton attributed— 
erroneously, moreover—the unequal solubility of the 
different gases to the circumstance that their ultimate 
particles are not equal in weight, and that the ele- 
mentary atoms of which they are formed are not equal 
in number. ; 

In this memoir he remarked that he had for some 
time been occupied with an endeavour to determine the 
relative weights of the ultimate particles of bodies—a 
new and, as he says, most important consideration. 
Without laying any special stress upon the development 
of these ideas, he gives in his memoir the first table of 
atomic weights as follows :— 


Hydrogen . ‘ : ° : ; 5 aed 
Oxygen : . : : . . . 2be 
Nitrogen. : , ° ° : s £2 


1 History of Chemistry, vol. li. p. 289. London, 1831. 


DALTON—HYPOTHESIS OF ATOMS. 25 


Phosphorus 

Sulphur 

Carbon . 

‘Water . 

Ammonia ., : 
Protoxide of nitrogen . 
Binoxide of nitrogen 
Nitric acid : ’ 
Phosphoretted hydrogen 
Sulphuretted hydrogen 
Sulphurous acid 
Sulphuric acid 
Carbonic oxide 
Carbonic acid 

Marsh gas 

Olefiant gas . 

Ether . 

Alcohol. 


7:2 
14-4 
4°3 
6°5 
5:2 
13:7 
9°3 
15°2 
8°2 
15-4 
193. 
25°4 
98 
15°3 
6°3 
53 
9°6 
15:1 


Let us first remark that not only the law of multiple 
proportions but also the atomic theory are clearly con- 


tained in this table. 


following data :-— 


4°3 of carton are com- 
bined with 1 of hydrogen in 5:3 of olefiant gas. 


4-3 ” ” 2 

4:3 ” ” 

4-3 a 2 Roe 
14-4 of sulphur 4 55 
14-4 » » 2x55 

4:2 of nitrogen __,, 55 

4°2 3 wie aa x 5:6 
Sx42 55 s 55 


” 


” 


This result is evident from the 


6°3 of marsh gas. 


5:5 of oxygen in 9°8 of carbonic oxide. 


15°3 of carbonic acid. 

19°9 of sulphurous acid. 

25-4 of sulphuric acid. 

9-7 (9:3) of binoxide of 

nitrogen. 

15°2 of nitric acid. 

13°9 (13°7) of protoxide 
of nitrogen, 


It is true that these figures are very inaccurate, but 
the inaccuracy of the numerical data cannot conceal or 


26 THE ATOMIC THEORY. 


diminish the grandeur and simplicity of the theoretical 
conception. 

Dalton here regards chemical combinations as 
formed by the addition of elementary atoms, the rela- 
tive weights of which he endeavours to determine, 
referring these weights to one of the elements—hydrogen 
—asunity. Whentwo bodies combine in several propor- 
tions, the combination can only be effected by the addi- 
tion of entire atoms: it follows, therefore, that, the 
proportion of the one body remaining constant, the 
proportions of the second must be exact multiples of 
each other. It is, then, clear that as early as the year 
1803 or 1804 Dalton had, if not formally stated, at least 
conceived and implicitly admitted the law of multiple 
proportions, as well as the atomic hypothesis, which 
may almost be regarded as the theoretical representa- 
tion of the fact of fixed and of multiple proportions. A 
few years afterwards he gave his opinion upon this 
subject in the following terms: !-— 

‘In all chemical researches great importance has 
with justice been attached to the determination of 
the relations according to which elements unite to 
form compound bodies; but, unfortunately, the subject 
has not been followed up, though the consideration of 
these relations might have led to important conse- 
quences concerning the relative weights of the smallest 
particles, or atoms, of bodies.’ 

From the year 1804 the atomic theory inspired all 
Dalton’s labours and influenced all his thoughts; he 
confesses himself the influence which this idea had upon 

1 A New System af Chemical Philosophy, part i. London, 1808. 


DALTON—HYPOTHESIS OF ATOMS. 27 


him in his representation of the constitution of marsh 
gas, which he was studying in 1804. It occasioned the 
discovery of multiple proportions, and afterwards fur- 
nished, by a happy reaction, a solid foundation for the 
atomic hypothesis. The latter was not, however, new 
to the epoch of Dalton. Not to speak of the atomists 
of the seventeenth century, who had revived, though at 
the same time distorted, the ancient conception of the 
Greek philosophers, we must not forget that Van 
Helment, N. Lemery, and Boerhaave had mentioned 
the indivisible particles of bodies, and had termed them 
‘atoms,’ and that Boyle had tried to explain the differ- 
ences between chemical attractions by the inequality of 
the ‘ massulz’ or particles. 

_ This was a correct conception: the ultimate parti- 
cles of bodies differ in their relative weight, and doubt- 
less in their size and form. In 1790 Higgins opposed 
this hypothesis, erroneously attributing the same weight 
to atoms which combine in very simple proportions to 
form compound bodies. Thus Higgins admitted that 
one ponderable part of sulphur in sulphurous acid is 
combined with one ponderable part of oxygen, and in 
sulphuric acid with two ponderable parts of oxygen, 
and that these compounds may be represented as con- 
sisting, the first of one atom of sulphur with one atom 
of oxygen, the second of one atom of sulphur with two 
atoms of oxygen, the atoms of these two elements being, 
moreover, of the same weight. He consequently re- 
presented these compounds as formed by the union of 
particles or atoms, of the same weight, but united in 
different proportions—one of nitrogen for two of oxygen 


28 THE ATOMIC THEORY. 


in binoxide of nitrogen, and one of nitrogen for five of 
oxygen in nitric acid. 

This is all perfectly clear, but the starting point is 
wrong. The proportions in which bodies combine do 
not represent equal weights. This was an established 
fact in the time of Higgins, and he is obliged to ac- 
knowledge it in the case of water, the two elements of 
which unite in very unequal proportions ; if therefore, 
as he admitted, water was composed of one atom of 
oxygen and one atom of hydrogen the atoms of these 
two elements could not be equal in weight. Higgins’s 
conception was, therefore, spoilt by errors and contradic- 
tions, and it is useless to attempt to represent him as one 
of the authors of the modern atomic theory. This honour 
belongs to Dalton alone. This great man began to 
consolidate and publish his views about the year 1808. 
Thomson and Wollaston were at that time developing 
the law of multiple proportions. In a* memoir upon 
oxalic acid Thomson showed that the acid oxalate of 
potash contains twice as much acid as the neutral 
oxalate. Wollaston demonstrated that this law applies 
to the quantities of bases and acids contained in basic 
and acid salts, these quantities bearing a simple ratio to 
each other. He showed that this is the case in the com- 
pounds of potash and soda with carbonic and sulphuric 
acids, and especially in the compounds of oxalic acid 
with potash. He points out that the latter are three in 
number, and that the quantities of acid which they 
contain for the same proportion of base increase as the 
numbers 1, 2, 4. 

At this time Dalton himself published his theory in 


DALTON’S NOTATION. 29 


the first part of his ‘ New System of Chemical Philoso- 
phy,’ which appeared in 1808. The new and compre- 
hensive idea of representing compound bodies as formed 
of groups of atoms, fixed in number, and possessing 
different, but at the same time fixed, relative weights, 
might, it seemed to him, be graphically expressed by 
the adoption of symbols representing these atoms, and 
grouped in such a manner as to indicate the composi- 
tion of bodies. Each atom was represented by a small 
circle bearing a particular sign. 

This is the origin of chemical notation, the lan- 
guage of symbols and numbers, which is clearer and 
more concise than that of words, and has since been a 
great instrument of progress in science and a great 
assistance in instruction. In the work mentioned above 
Dalton gives a new table of atomic weights, more com- 
plete and less incorrect than the preceding one. We 
give a few of these atomic weights. 


Dalton’s Correct 
Atomic Weights. Numbers. 

Hydrogen : : - ot 1 
Nitrogen . ° . . : ae 4°66 
Carbon . : ° : in a? EF 6 
Oxygen . E - : ER i 8 
Phosphorus . : ‘ a. o 10°3 
Sulphur . : : : were 16 
Iron. ° : ; : - 38 28 
Zinc. : . ° ° . 56 65°2 
Copper . . ; : - 56 64:5 
Lead d ; ‘ ‘ - 95 104 
Silver. ° “ ° - 100 108 
Platinum . ° ‘ ; - 100 98:5 
Gold : : ; ‘ . 140 197 


Mercury . ~ ‘ . « 167 200 


30 THE ATOMIC THEORY. 


We have omitted in this table the atomic weights 
of the alkalies and earths which are still placed among 
the elements, though Dalton must have already been 
acquainted with the great discovery of H. Davy upon 
the nature of the alkalies. The above figures give, 
however, a sufficiently good idea of the accuracy, or 
rather the inaccuracy, to which Dalton had attained in 
his own determinations, or in the discussion of those of 
others. At the same time they show us the exact sense 
in which we must regard these atomic weights. They 
are not, properly speaking, atomic weights in the sense 
which we now ascribe to the term; they are proportional 
numbers referred to unity, which represents the weight 
of hydrogen in hydrogen compounds. ‘This may be seen 
from the following table, in which, for the sake of brevity, 
we have employed the symbuls in use at the present 
day :— 


Atomic 
Weights. 
Water contains 1 at. H, which weighs 1, and 1 at. O, which 
weighs 7 . ° 8 
Sulphuretted hydrogen Soran 1 at, TH, hice we} srg ib aa 1 
at. 8, which weighs 13 - : 14 
Ammonia contains 1 at. H, which weighs 1, se 1 “ N, milion 
weighs 5 : ; 6 
Olefiant gas contains 1 at. H, rainth wens a; ve 1 eh C, Poe 
weighs 5 * 3 6 
Marsb gas contains 2 at. H, ian wah 2, ani 1 fe C, hich 
weighs5 . =f 
Carbon protoxide contains 1 at. 0, which meee 5, BN 1 = 0, 
which weighs 7 . 3 12 
Carbonic acid contains 1 at. C, eich Hei? 5, oa 2 Ja 0, 
which weigh 14 . ‘ 19 
Protoxide of nitrogen contains 9 at. N, hich Loigt LO; a 1 
at. O, which weighs 7 . ee ‘ 2 . ; - te 


ES 


DALTON’S NOTATION. 31 


Atomic 
; ‘ Weights. 
Binoxide of nitrogen contains 1 at. N, which weighs 5, and 1 at. 
O, which weighs 7 ‘ he 
Nitrous acid contains 2 at. N, hich weld 10, aid 3 af O, 
weigh 21. : » ol 
Nitric acid contains 1 at. N, which Soinhe 5, Sie 2 ve O, which 
weigh 14, - . : : p - : : 19 


We see that the atomic weights of oxygen, sulphur, 
nitrogen, carbon, and phosphorus are deduced from the 
composition of their combinations with hydrogen, in 
which the existence is admitted of one atom of hydro- 
gen combined with one atom of another body; and when 
there are two combinations with hydrogen, as is the 
case with carbon, the atomic weight is determined from 
that containing the least quantity of hydrogen. Thus 
the atomic weight of carbon is the quantity of carbon 
combined with 1 of hydrogen in olefiant gas. In 
marsh gas this quantity of carbon is combined with 2 of: 


~ hydrogen. 


Such are the principles by which Dalton was guided 
in the determination of atomic weights, as they were 
conceived by him in 1808, and in the notation which 
was deduced from them. These principles are clearly 
demonstrated in the following table, which expresses the 
atomic constitution of the compounds mentioned above; 
the formule are analogous to those now in use :— 


Dalton’s Notation (1808). 


Atomic 
(Molecular) . 
Weight. Formule. 
8 of water are represented by . ; a. HO 
14 ,, sulphuretted hydrogen ; F . HS 
6, ammonia . ‘. r : ; a: SELES 


6 ,, olefiant gas . : ‘ . ‘ « HO 


32 


THE ATOMIC THEORY. 


Dalton’s Notation (1808)—continued. 


Atomic 
(Molecular) = 
Weight. 
7 of marsh gas 


12 
19 
17 
12 
31 
19 


», carbon protoxide . 

5 carbonic acid 

», protoxide of nitrogen 
», binoxide of nitrogen 
») nitrous acid . . 

» Ditricacid . ‘ 


Formule. 
2 


co 
co, 
N,0 
NO 
N,0, 


_ NO, 


CHAPTER II. 


LAW OF VOLUMES. 


GAY-LUSSAC—AVOGADRO AND AMPERE—BERZELIUS. 


1p 


TuE atomic weights established by Dalton were really 
proportional numbers; they represented the proportion 
in which bodies combine, expressed by the relative 
weights of their ultimate particles. The atoms of 
simple bodies are equivalent to each other. We may, 
therefore, consider the terms atomic weights, propor- 
tional numbers, and equivalents as at this time syno- 
nymous. We owe the last term to Wollaston; H. Davy 
preferred the expression ‘ proportional numbers.’ 

The atomic constitution of bodies follows very 
naturally from the ideas of Dalton. In binary com- 
pounds atoms unite in the ratio of 1 to 1, and in 
multiple compounds formed by two given elements in 
the ratio of 1 to 1,1 to 2,1 to 3,2 to3, &. This simple 
conception, which is clearly demonstrated in the table 
upon the preceding page, had to be modified in accord- 
ance with Gay-Lussac’s great discovery. 


34 THE ATOMIC THEORY. 


The relations between the combining volumes of 
gases are very simple, and the volume of the compound 
formed bears, moreover, a very simple ratio to the sum 
of the volumes of the combining gases. 

This proposition embraces a great number of facts, 
which present no exceptions and which together consti- 
tute a great law of nature, the law, namely, of Gay- 
Lussac. Suitably interpreted, it has become one of the 
foundations of chemical science. ‘The following are the 
facts; the interpretation will be developed presently :— 


2 vol. of hydrogen unite with 1 vol. of oxygen to form 2 vol. of 
aqueous vapour.! 

2 vol. of nitrogen unite with 1 vol. of oxygen to form 2 vol. of 
nitrogen protoxide. 

1 vol. of nitrogen unites with 1 vol. of oxygen to form 2 vol. of 
nitrogen dioxide. 

1 vol. of nitrogen unites with 2 vol. of oxygen to form 2 vol. of 
nitrogen peroxide. 

1 vol. of chlorine unites with 1 vol. of hydrogen to form 2 vol. of 
hydrochloric acid gas. 

2 vol. of chlorine unite with 1 vol. of oxygen to form 2 vol. of 
hypochlorous anhydride. 

1 vol. of nitrogen unites with 3 vol. of hydrogen to form 2 vol. of 
ammonia. | ; 

2 vol. of carbon protoxide unite with 2 vol. of chlorine to form 2 
vol. of phosgene gas. | 

2 vol. of ethylene unite with 2 vol. of chlorine to form 2 vol. of 
vapour of ethylene chloride, 


Thus it appears that very simple relations exist 
not only between the volumes of gases entering into 
combination, but also between these volumes and the 
volume occupied by the gas or-vapour of the com- 

1 The volumetric composition of water WAS discovered in 1805 


by Gay-Lussac and Humboldt. This observation formed the 
starting point of Gay-Lussac’s discoveries, 


GAY-LUSSAC—LAW OF VOLUMES. 35 


pound body. It should be remarked, moreover, that, as 
far as we know at present, the volumes of the combining 
gases are always reduced to 2 vol. after combination." 
Bearing this fact in mind, we may return to our his- 
torical account. } 

Gay-Lussac rendered unexpected assistance to the 
ideas of Dalton. The fixed relations which are ad- 
mitted between the weights of elements entering into 
combination, the simple relations which exist between 
the weights of a given element in the multiple combina- 
tions of that element, are again encountered when the 
combining volumes of gases are considered. Connecting 
these two orders of facts, and following up the interpre- 
tation which Dalton gave of the former, may we not 
conclude that the relative weights of the gaseous 
volumes entering into combination exactly represent 
the relative weights of the atoms—in other words, that 
there exists a simple relation between the specific 
gravities of elementary gases and their atomic weights ? 
Gay-Lussac perceived this simple relation, and Berzelius 
defined it a few years afterwards; but Dalton refused to 
accept it, ignoring and repudiating the solid support 
which the great French chemist gave to his ideas. 

In fact, the relation which exists between the 
densities of gases and their atomic weights is not so 
simple as we should at first sight be led to expect, and 
as for a long time it was thought to be. 

It is a difficulty which will soon be apparent, and 


1 This applies particularly to the first seven cases, in which the 
-volumetric relations are as simple as possible, and cannot be 
reduced. The two last cases will be discussed presently. 


36 THE ATOMIC THEORY. 


which has only quite recently been overcome, after sixty 
years of investigation and labour. Nevertheless the 
theoretical conception which embraces the two orders 
of phenomena in question, and which establishes a link 
between fixed and multiple chemical proportions and 
the law which regulates the combinations of gaseous 
volumes, was accurately formulated in 1813 by the 
Italian chemist Amedeo Avogadro. 

Starting from the discoveries of Gay-Lussac, Avoga- 
dro arrived at the conclusion that there exists a simple 
relation between the volumes of gases and the number 
of elementary or compound molecules which they con- 
tain. The most simple and at the same time the most 
probable hypothesis which can be brought forward upon 
this point is, he says, to admit that all gases contain in 
a given volume the same number of integral mole- 
cules.!| These molecules must, therefore, be equi- 
distant from each other in ditferent gases, and placed 
at distances which, in relation to the dimensions of the 
molecules, shall be exactly sufficient to neutralise their 
mutual attraction. This hypothesis, according to 
Avogadro, is the only one which gives a satisfactory 
explanation of the fact of the simplicity of relations 
between the volumes of gases entering into combina- 
tion. The following result of this hypothesis is im- 
portant: if it is true that equal volumes of gases contain 
the same number of molecules, the relative weights— 
that is to say, the densities of equal volumes—ought to 
represent the relative weights of the molecules. Thus 
the molecular weights of hydrogen, oxygen, and nitro- 


1 Journal de Physig e, vol. xxxili. p. 58. 


HYPOTHESIS OF AVOGADRO. ST 


gen will be expressed by the ratio of their densities 
—i.e. 1, 15, 13.1 But in considering the molecular 
weights of compound bodies we encounter a difficulty, 
which arises from the difference of contraction experi- 
enced by gases in the act of combination. Supposing 
water to be formed by the union of two volumes of hydro- 
gen and one volume of oxygen contracted to one volume, 
it is clear that the weight of this single volume, com- 
pared with that of one volume of hydrogen, would 
be 17 (15+2);? or again, supposing one volume of 
. ammonia were formed by the contraction of three 
volumes of hydrogen and one volume of nitragen, the 
weight of this volume of ammonia must be 16. Now, 
experiment proves that the densities of aqueous 
vapour and of ammonia are half the above numbers 
—namely, 8°5 and 8—a result which agrees with the 
fact that two volumes of hydrogen and one volume 
of oxygen are condensed into two volumes of aqueous 
vapour, and, on the other hand, that three volumes of 
hydrogen and one volume of nitrogen are condensed 
into two volumes of ammonia. Since one volume of 
aqueous vapour contains only one volume of hydrogen 
and 5 volume of oxygen, a molecule of water can only 
be formed of one molecule of hydrogen and 4 molecule of 
oxygen ; and, for the same reason, one molecule of ammo- 
nia must be formed of 14 molecule of hydrogen and 4 
molecule of nitrogen, and a molecule of hydrochloric 
acid gas of $ molecule of hydrogen and 4 molecule of 
chlorine. It follows that the matter contained in the unit 


1 The correct numbers are 1, 16, 14. 
? These numbers are those of Avogadro, 


38 THE ATOMIC THEORY. 


of volume of the elementary gases does not represent 
the ultimate particles which exist in certain combinations 
of these gases, for the matter contained in one volume 
of oxygen, nitrogen, hydrogen, and chlorine must be 
halved in order to form the quantity of water, ammonia, 
and hydrochloric acid gas contained in one volume of — 
these compounds. This is the difficulty which strikes 
us, a difficulty which, according to-Avogadro, is easily 
solved by supposing that the wntegral molecules, an equal 
number of which are contained in the gases or vapours 
of elementary bodies, are themselves composed of a 
certain number of elementary molecules of the same 
kind, just as the antegral molecules of compound gases 
and vapours are formed of a certain number of element- 
ary molecules of different kinds. 

This is a fundamental idea, and at that time was 
quite new. Whilst Dalton had only distinguished one 
kind of ultimate particles—atoms—Avogadro admits 
the existence of two kinds—an important distinction, 
which has been established by the progress of science. 
Avogadro’s elementary molecules are atoms, while 
the integral molecules, which are equidistant from 
each other in gaseous bodies, which are set in motion 
by heat and excited by affinity, are what we at the present 
time call molecules. 

Ideas similar to those of the Italian chemist were 
published in 1814 by Ampére,' who established a distine- 

1 A letter from M. Ampére to M. le Comte Berthollet upon the 
determination of the proportions in which bodies combine, from 
the respective number and arrangement of the molecules of which 


their integral molecules are composed. (Annales de Chimie, vol. 
xe. p. 43.) 


HYPOTHESIS OF AMPERE. 39 


tion between particles and molecules. The particle, 
he says, ‘is a collection of a definitive number of mole- 
cules in a definite situation, occupying a space incom- 
parably greater than that of the volume of the molecules.’ 
And he adds, ‘ When bodies pass into the gaseous state, 
their several particles are separated, by the expansive 
force of heat, to much greater distances from each 
other than when the forces of cohesion or attraction 
exercise an appreciable influence, so that these distances 
depend entirely upon the heat to which the gas is sub- 
jected, and that, under equal pressure and temperature, 
the particles of all gases, whether simple or compound, 
are equidistant from each other. The number of par- 
ticles is, on this supposition, proportional to the volume 
of the gases.’ This passage is so remarkable that we have 
quoted it word for word. But, as a natural consequence 
of Ampére’s proposition, it follows that, the distances 
between the gaseous particles being the same, and 
depending solely upon pressure and temperature, the 
same variations of pressure and temperature should 
produce the same change of volume in the gases. 

This, as we know, is actually the case, and this 
great physical fact of the sensible equality of the 
expansion of gaseous volumes follows as the result of the 
principle propounded by Avogadro and Ampére —namely, 
the equality of the number of particles contained in 
equal volumes of gases and the equality of the distances 
by which they are separated. Neither of them laid any 
stress upon this result, which clearly supported their 
hypothesis. 

Ampére clearly alludes to it in the passage quoted 


40 THE ATOMIC THEORY. 


above, but he only adds that if the hypothesis which 
he has offered ‘agrees with the established results of 
experiment, and if such results can be deduced from it 
as will be confirmed by subsequent experiments, it may 
acquire a degree of probability approaching to what in 
physics is termed certainty.’ 

Thus, equal volumes of gases and vapours contain 
the same number of particles, and the latter are formed 
of groups of molecules. This, in other words, is the con- 
ception of Ampere. From geometrical considerations 
Ampére was led to conclude that each particle consists 
of four molecules. 

‘In accordance with this idea,’ he says, ‘ each 
particle should be regarded as a collection of a definite 
number of molecules in a definite position, occupying a 
space incomparably greater than that of the volume of 
the molecules; and, in order that this space may be of 
three comparable dimensions, one particle must comprise 
at least four molecules. In order to express the respec- 
tive position of the molecules in a particle, we must 
conceive, by means of the centre of gravity of these 
molecules, to which we may suppose them to be reduced, 
planes so placed as to leave on the same side all the 
molecules which are outside each plane. Supposing 
that no molecule should be contained in the space 
included between these planes, this space will be a 
polyhedron, of which: each molecule will occupy an 
angular point. 

If the particles of oxygen, nitrogen, and hydrogen 
are composed of four molecules, it would follow, accord- 
ing to Ampére, that those of nitrogen dioxide are 


HYPOTHESIS OF AMPERE. 41 


composed of four molecules—namely, two of oxygen and 
two of nitrogen—and those of nitrogen protoxide of 
six molecules—namely, four of nitrogen and two of 
oxygen. 

Thus when gases combine together the molecules 
contained in the unit of volume of either of the com- 
bining gases, and which form its particle, are not always 
contained integrally in the unit of volume of the com- 
pound gas. These molecules may be grouped or 
separated in forming a particle of the compound gases. 
Let us translate this idea of Ampére’s into the language 
of formule, confining ourselves to the examples quoted 
above. 

The unit of volume of gas contains— 


Oxygen . : - : : : aamye 
Hydrogen : ‘ - 3 :  welig 
Nitrogen . " ‘ : : : aly 
Water. ‘ ‘ ° . , » H,O, 
Nitrogen protoxide ; : : ae PoE 
Nitrogen dioxide . : . ‘ - N,O, 
Ammonia . . : ° ° Ee N*HG 


The analogy will at once be seen between this 
conception and that which is generally adopted at the 
present day, and which is expressed by the following 
formule :-— 

Two volumes of gas contain— 


Oxygen . ; 3 , ‘ ; Ls Od 
Hydrogen , : é . ° wie EL 
Nitrogen ’ . ; : fpr Re3 
Water - ; 3 ‘ : : , - HO 
Nitrogen protoxide ; ‘ : « N,O 
Nitrogen dioxide . ‘ : 2 a WO 
Ammonia ° ‘ ‘ 3 : - NH; 


3 


42 THE ATOMIC THEORY. 


For the first time gases are compared under the . 
same volume, and the matter contained in equal volumes 
represents the magnitude of the molecules, which is 
a most essential point. The integral molecules of 
Avogadro and the particles of Ampére.are, in fact, the 
material parts contained in the unit of volume. These 
integral molecules, or particles, may be subdivided 
into elementary molecules, or simply into molecules ; and 
Ampére, from geometrical considerations, ingeniously, 
though uselessly, multiplies the number of the latter. 
Both for the first time introduce into science the dis- 
tinction between two kinds of ultimate particles, and 
admit that the number of integral molecules or particles 
is proportional to the gaseous volume. 

We now give a more simple form to the same ideas 
by admitting that gases (and all other bodies) are 
formed of molecules and atoms; and, in order to avoid 
the subdivision of molecules referred to one volume, we 
find it more convenient to refer them to two volumes, 
assigning the term molecule to the matter contained in 
two volumes. When for an elementary gas this molecule, 
as is often the case, consists of two atoms, the atom 
represents the matter contained in one volume; but the 
general rule is that equal volumes of gases and vapours 
contain the same number of molecules, and conse- 
quently that the relatwe weights of these molecules are 
proportional to the densities. This is the law, or, if 
we prefer it, the hypothesis, of Avogadro and Ampére, | 
for we must acknowledge that a hypothesis here becomes 
mingled with the interpretation of positive facts ; but the 
hypothesis seems to be legitimate, and will be justified 


INTERPRETATION OF BERZELIUS. 43 


presently. Chemists long ignored its import. Another 
conception was soon substituted for it, which acquired 
an important place in science, without, however, gaining 
general consent, though supported by the authority of a 
great name—that of Berzelius. ? 
Hae is 

In 1813 Berzelius conceived the idea that, in order 
to represent the composition of bodies, we must take 
into consideration the relative volumes in which simple 
gases combine to form compound gases. He developed 
this idea in a memoir upon the nature of hydrogen. 
It is well known, he says, that one volume of a gaseous 
body combines with one, two, or three volumes of 
another gaseous body; we have only to determine, 
therefore, the weights of these volumes to know the 
relative weights according to which the gases combine 
with one another. It is obvious that the numbers thus 
obtained are similar to the atomic weights of Dalton, 
without, however, being identical. Again, though 
declaring himself a partisan of the atomic hypothesis, 
Berzelius held that it was better to keep to the theory 
of volumes, as having the advantage of being founded 
on well-established facts. In order to express the com- — 
position of bodies by weight from this point of view, it — 
is only necessary to find:—firstly, the number of element- 
ary volumes which unite to form a compound body; 
secondly, the relative weights of these volumes—that is 
to say, the densities of the elements in the gaseous 
state. Hence the importance of the determination of 
gaseous densities. In 1814 Berzelius worked out a 


44 THE ATOMIC THEORY. 


number of these densities, referring them to that of 
oxygen, which he took as 100. 

Thus, the weight of an ‘elementary volume’ of 
oxygen being 100, 
that of an elementary volume of hydrogen will be 
6:218, 
that of an elementary volume of nitrogen 88°6, 
that of an elementary volume of chlorine 221°4.! 

These weights express the quantities of the bodies 
which enter into combination. In a great number of cases 
there are many difficulties in the way of the determina- 
tion, which can only be made in an indirect manner. 
In fact, at this period Berzelius was only acquainted 
with two simple gases of-which he could obtain the 
density—oxygen, namely, and hydrogen. He at that 
time regarded nitrogen as a body composed of nitrogen 
and oxygen, and was not as yet converted to Davy’s 
opinion concerning the simple nature of chlorine. 

It would, therefore, be only in a very small number 
of cases that the weight of ‘ elementary volumes’ could 
be directly proved by experiment, and to determine the 
. weight of non-gaseous simple bodies we should be forced 
to have recourse to hypotheses upon the composition by 
volumes of non-gaseous elements. Let us take a few: 


1 We can judge of the accuracy of these numbers by referring 
them to 6-218 of hydrogen taken as unity. They then bear the 


following ratios to each other :— 
Correct 


Numbers. 
Hydrogen : . . oi Bs. 1 
Oxygen . A ; : . 16:08 16 
Nitrogen : ‘ : . 14°45 14 


Chlorine . 5 Ny . 361) 35°5 


INTERPRETATION OF BERZELIUS. 45 


examples. What is the weight of an ‘elementary 
volume’ of carbon? We know that carbonic acid gas 
contains its own volume of oxygen. But do the two 
volumes of carbonic acid gas, which contain two volumes 
of oxygen, contain one volume or two volumes of carbon 
vapour? In the first case the three volumes, two of 
oxygen and one of carbon vapour, are reduced to two 
from the effect of combination, a condensation similar 
to that of water; in the second the condensation is 
one-half. Thus, on the first hypothesis, it is evident 
that the weight of the ‘elementary volume’ of carbon 
is twice that which is attributed to it in the second. 
Referred to oxygen as 100, the weight of the ‘ element- 
ary volume’ of carbon is 75:1 in the first case and 
37:55 in the second; and the corresponding formulx 
of carbonic acid gas will be CO, and ©,0,. Berzelius 
adopted the first hypothesis, allowing himself to be 
guided by analogy. It seemed to him probable that the 
condensation of the elements of carbonic acid gas was 
similar to the condensation of the elements of water. 

He also at this time admitted that the powerful 
bases must be composed of two elementary volumes of 
oxygen and one volume of metal. The composition of 
the oxides of sodium, potassium, calcium, iron, zinc, and 
lead was, therefore, represented by the formule Na0O,, 
KO,, CaO,, FeO,, PbO,, the weights of the element- 
ary volumes of a great number of metals thus assuming 
a value double that which Berzelius attributed to them 
later. 

The theory of volumes, as it stood at that time, was 
therefore bristling with hypotheses and full of uncer- 


46 THE ATOMIC THEORY. 


tainties. And yet this conception long held its ground 
in science, especially in France, where at a certain period 
it was the fashion to express the composition of bodies 
in § volumes,’ under the impression that the substitution 
of volumes for atoms had the advantage of offering a 
representation more in accordance with facts. But in 
reality it was not so: the volume occupied by carbon 
vapour, and the degrees of condensation of the elements 
of carbonic acid gas, were hypothetical ideas, and these 
‘elementary volumes’ represented the atoms themselves, 
at least in notation. 

Berzelius recognised this fact in 1818. In his essay 
upon the theory of chemical proportions he modified 
considerably the views which he had published in 1813. 
The prevailing idea is no longer that of establishing the 
system of atomic weights upon the theory of volumes. 
Though still giving weight to the indications furnished 
by this theory, he endeavours to reconcile it with what 
he terms the ‘corpuscular theory,’ which is founded 
upon chemical proportions. The indivisible corpuscules, 
or the ultimate particles of bodies, are designated 
atoms—the most convenient term, because the one most 
in use. We might call them particles, molecules, or 
chemical equivalents, as all these terms appear to be 
synonymous to Berzelius. The relative weights of these 
atoms represent chemical proportions. The fixed pro- 
portions, which had been recognised for weights, again 
appeared in gaseous combinations for volumes. Thus 
the theory of volumes and the atomic or corpuscular 
theory led to the same results, as far as the ponderable 
relations of elements in combinations are concerned: 


BERZELIUS—CORPUSCULAR THEORY. 47 


what is called atom in one is called volume in the other. 
It would seem, therefore, as if we might assimilate the 
two notions, which indeed is necessary in the case of 
simple gases. Equal volumes of the latter contain the 
same number of atoms, under the same conditions of 
temperature and pressure. Berzelius observes that this 
law does not apply to compound gases; for, he says, it 
sometimes happens that a volume of a compound gas 
contains fewer atoms than an equal volume of a simple 
gas. Thus one volume of aqueous vapour contains one- 
half as many atoms (compound atoms, molecules) as 
one volume of hydrogen. 

Such was the manner in which Berzelius, about 
1818, expressed the atomic hypothesis, which he founded 
partly upon chemical proportions and partly upon a 
peculiar conception of the law of volumes. This con- 
ception was not a very happy one. Not to mention the 
difficulty which he created by applying the same term, 
atoms, to the ultimate indivisible particles of simple 
bodies and to the complex molecules of compound 
bodies, a confusion which had been avoided by Avogadro 
and Ampére, Berzelius at this time introduced into 
the language of science a formula which long held 
its ground, and which must now be considered as 
erroneous—namely, the proposition that equal volumes 
of simple gases contain the same number of atoms. 
We shall presently reconsider this point. We must 
here draw attention to the influence which the discoveries 
of Gay-Lussac exercised upon Berzelius in his attempt 
to bring the atomic hypothesis into harmony with the 
facts relating to the combination of gases. It is a 


48 THE ATOMIC THEORY. 


remarkable fact that neither Dalton nor Gay-Lussac 
accepted the views of the Swedish chemist. The author 
of the atomic theory obstinately maintained his first 
idea of deducing atomic weights solely from the 
ponderable relations of elements in combinations. Gay- 
Lussac, again, confined himself to the immediate con- 
sequences of his discovery, not without forcing them to 
some extent, in certain cases, by hypotheses upon the 
forms of condensation of the combining gaseous elements. 
He and Berzelius expressed the composition of bodies 
in volumes, the latter admitting that the relative 
weights of these volumes represented atoms, Gay-Lussac 
refusing to consider these weights as anything more than 
ponderable ‘ relations,’ and inclining rather to the views 
of Davy. The latter, deviating to an equal extent from 
the profound conceptions of Dalton, and with the idea 
of completing them by the discoveries of the French 
chemist, confined himself strictly to established facts 
and to the consideration of ‘proportional numbers.’ 
After the ingenious but ignored attempts of Avogadro 
and Ampére, and the unfruitful effort of Berzelius, 
Dalton’s conception would have been sentenced to 
sterility and oblivion, had it not happened that, at the 
period of which we are speaking, fresh discoveries and 
new ideas drew attention to it. We allude to Prout’s 
hypothesis, to the discovery of the law of specific 
heats, and to the discovery of isomorphism. 


CHAPTER III. 


PROUTS HYPOTHESIS—LAW OF SPECIFIC IIEATS— 
ISOMORPHISM. 


DULONG AND PETIT—MITSCHERLICH. 


Ee 


We must first return to Prout’s hypothesis, not that it is 
of such great importance from our present point of view, 
but because it preceded the important discoveries which 
we shall presently mention. 

The anonymous author of a memoir which appeared 
in 18151 upon the relations between the densities of 
bodies in a gaseous state and the weights of atoms, tried 
to prove that the densities. of oxygen, nitrogen, and 
chlorine are integral multiples of that of hydrogen, and 
that the atomic weights of certain elements are similarly 
integral multiples of that of hydrogen. Amongst these 
elements we meet with some metals, the atomic weight 
of which had been determined by the author or by other 
chemists by the following excellent process: the quan- 
tities of metal were determined which, combined with 
- oxygen, formed quantities of oxides capable of neutra- 


1 Annals of Philosophy, vol. vii. p. 111. 


50 THE ATOMIC THEORY. 


lising the same quantity of an acid. The results appear 
in the following table :— 


H= 1 Ca = 20 
= 6 Na=24 
N= 14 Fe=28 
P= 14 Zn = 32 
O- 8 K =40 
S = 16 Ba=70 
Cl= 36 
I =124 


We will make no remark upon the author’s con- 
siderations concerning the relations which may be traced 
between these numbers and those whichexpressthe atomic 
weights of other elements determined with less accu- 
racy. These considerations were obscure and erroneous. 

The important point was raised by Prout in 1816, 
in a work to which he appended his name. ‘ It is very 
advisable,’ he remarked, ‘to adopt the same unit for 
specific weights and atomic weights, and to take as this 
unit the weight of one volume of hydrogen. The same 
numbers will thus give the densities of gases and the 
atomic weights, or a multiple of these weights. If, 
proceeds the author, ‘ these numbers are whole numbers, 
the fact under consideration may be interpreted by ad- 
mitting that hydrogen is the primordial matter which 
forms the other elements by successive condensations. 
The figures expressing these condensations—that is to 
say, the densities—would at the same time give the 
number of volumes of primordial matter condensed into 
a single volume of a given element, and the weight 
of this volume, expressed by a whole number, would 
represent the atomic weight of the element.’ 


PROUT’S HYPOTHESIS. 51 


The determinations of atomic weights, aud even 
those of the densities of gases, were too inaccurate, at 
the time of which we are speaking, for Prout’s hypothe- 
sis to be taken into serious consideration. It was a 
conjecture. It has,as we know, been lately again taken 
up with great energy by Dumas.! 

But the accurate determinations of a number of 
atomic weights by Stas, notably those of chlorine, 
potassium, sodium, and silver, by confirming or slightly 
rectifying the results formerly obtained by Marignac, 
have entirely annihilated the celebrated hypothesis in 
question. Unsuccessful attempts have been made to 
revive it, by taking as the unit, not the atomic weight 
of hydrogen, but the half or the quarter of this weight. 
There are well-known atomic weights, particularly that 
of potassium, which are not a multiple of the fraction 3, 
nor even of 4. If, however, we retained the idea, which is, 
moreover, striking and profound, of a primordial matter 
the sub-atoms of which were grouped in different num~ 
bers to form the chemical atoms of hydrogen and the 
various simple bodies, and attributed to these sub-atoms 


1 Dumas made a communication to the Académie des Sciences 
(January 14, 1878) relative to the atomic weight of silver, dis- 
cussing the error which had arisen in the determination of this 
atomic weight, from the property which metallic silver possesses 
of retaining about ;2,; of oxygen, if the latter has not been care- 
fully expelled by heating it in vacuo to 600° C. Dumas main- 
tains the number 108, which he had previously adopted. He 
remarks that other atomic weights, described as forming excep- 
tions to Prout’s hypothesis, might probably be included in 
the rule, if, in the process of weighing, account were taken of 
errors similar to those which he had pointed out in the case of 
silver. 


UNIVERSITY OF ILLINOIS 
LIBRARY 


52 THE ATOMIC THEORY. 


a weight inferior to a quarter of that of hydrogen, 
assigning to it, for example, +, of that weight, such an 
hypothesis would, I say, escape all experimental verifi- 
cation, as the differences which it would then be our 
object to establish between the atomic weights of the 
various simple bodies would fall within the limit of 
errors of observation. Such an hypothesis, though rea- 
sonable, ceases to be legitimate, and positive chemistry 
for the present must abandon this theory of Prout’s, 
this dream of the ancients upon the unity of matter 
and the compound nature of chemical elements. Does 
this mean that Prout wrote in vain? By no means. 
His idea gave rise to valuable work and important dis- 
cussions, and the example which he set of referring 
atomic weights to that of hydrogen has been followed, 
for all chemists have adopted this unit. But at that 
time, as in our own, Prout’s conception produced no 
argument in favour of the atomic theory, and has 
exercised no influence upon the development of that 
theory. 


1Ale 


The discovery which we are about to mention is, on 
the contrary, one of the foundations of the atomic theory. 
It draws our attention to the relation which exists be- 
tween the atomic weights and the specific heats of solid 
elements. In a memoir published in the ‘ Annales de 
Chimie’ in 1819, Dulong and Petit gave the specific 
heats of a great number of solid bodies, particularly 
metals, and remarked that these specific heats were 
generally inversely proportional to the atomic weights. 


DULONG AND PETIT. 53 


Their observations are summed up in the following 
table :— 


| Product of the | 
i Relative Weight of each Atom 
Weights of multiplied by the 
| Atoms.* | corresponding 
Capacity. : 
| 
pester aes tthe | 
Bismuth .. 0°0288 13:30 | 0°3830 | 
Lead i - 0°0293 139624 0°3794 
Gold ; : 0°0298 12°43 03704 
Platinum . ‘ 00314 11°16 0°3740 
Tim) |, ‘ ; 00514 7°35 0°3879 
Silver : - 0°0557 6°75 0°3759 
Zine. : ; 0:0927 4-03 0°3736 
Tellurium. . 0°0912 4°03 0°3675 
Nickel = r 0°1035 3°69 0°3819 
Iron . A : 0°1100 3°392 O°3731 
Cobalt. . 0°1498 2-46 03685 | 


Sulphur .. 0:1880 2-011 | 0°3780 


This table contained several errors, which were cor- 
rected by V. Regnault. Thus the specific heats attri- 
buted to tellurium and cobalt were much too high. On 
the other hand, the atomic weights adopted for these 
elements were too low. 

Disregarding these inaccuracies, which were after- 
wards corrected, we find that the atomic weights 
adopted by Dulong and Petit for a large number of 
metals differ materially from those admitted at the same 
period by Berzelius. The atomic weights of zinc, iron, 
nickel, copper, lead, tin, gold, and telluritim were half 
those which Berzelius attributed to the same metals 


1 Referred to that of oxygen taken as unity. To compare these 
numbers with those of Berzelius on p. 62, it will therefore only 
be necessary to multiply them by 100. 


54 THE ATOMIC THEORY. 


in their principal oxides, which he then erroneously 
regarded as dioxides.' 

But Dulong and Petit justly remark that the ordi- 
nary methods of determining atomic weights by chemi- 
cal proportions often give a choice between several 
numbers. ‘There is always, they say, ‘something 
arbitrary in the determinations of the specific weight of 
elementary molecules (atomic weights); but the un- 
certainty only rests between two or three numbers, 
which always bear to each other a very simple ratio.’ 
In this case we should prefer the number which agrees 
with the law of specific heats. Moreover, the determi- 
nations adopted by Dulong and Petit are in accordance 
with the most firmly established chemical analogies. 
They only apply to a limited number of simple bodies. 
‘Still the mere inspection of the numbers obtained 
points to a relation so remarkable in its simplicity as to | 
be at once recognised as a physical law, susceptible of 
being generalised and extended to all elementary sub- 
stances. In fact, the products in question, which 
express the capacities for heat of atoms of different 
nature, are so nearly the same for all, that we cannot 
but attribute these very slight differences to inevitable 
errors, either in the determination of capacities for heat 
or in the chemical analyses.’ This is quite true; the 
errors have been corrected, and the exceptions disap- 


1 Tf. we substitute for the double atomic weights of Berzelius 
those which Dulong and Petit calculated from the specific heats, 
the following formulz of Berzelius, ZnO, FeO,, NiO,, CuO,, PbO,, 
SnO,, AuO,, will become ZnO, FeO, NiO, CuO, PbO, SnO, AuO. 
That the latter are correct Berzelius did not hesitate to ac- 
knowledge. 


LAW OF SPECIFIC HEATS. 55 


peared one by one, But this result was only obtained 
upon the completion in some points of the great re- 
searches commenced in 1840 by V. Regnault upon 
specific heats, which has only very recently been accom- 
plished. A reform in the system of atomic weights was 
also necessary, a reform which has taken place slowly 
and by degrees. 

Dulong and Petit recognised the importance of 
their discovery and did not exaggerate it. They 
brought to light a great law of nature, which they 
expressed in the following striking form: ‘The atoms 
of all simple bodies have precisely the same capacity 
for heat.’' This simple statement was of the greatest 
value to the idea of atoms, which until then rested 
upon purely chemical considerations, for we here meet 
with a physical relation between atoms, to which another 
physical relation was soon added—that, namely, existing 
between the density of gaseous bodies and the weight of 
their ultimate particles. In both cases the true formula 
had been wanting. Dulong and Petit readily discovered 
it as far as concerns the specific heats of atoms ; Berze- 
lius had been less fortunate in his attempt to define the 
volumes occupied by the latter in gases. 


III. 


Those were times of great activity, and the fertile 
discovery which we have just mentioned was soon fol- 
lowed by another, which exercised a manifest influence 


1 Loe. cit., p. 405. 


56 THE ATOMIC THEORY. 


upon the development of chemical theories. In the 
month of December 1819 E. Mitscherlich made known 
the law of isomorphism. The experiments which he 
was making upon phosphates and arsenates led to the 
discovery. He first established the fact that these salts 
resemble each other in composition, if this composition: 
is represented in ‘ proportions,’ phosphoric acid consisting 
of one proportion of phosphorus and five proportions of 
oxygen, and arsenic acid of one proportion of arsenic 
and five proportions of oxygen. This being granted, he 
observed, further, that the phosphates and arsenates of 
the same bases, combined with the same quantity of 
water of crystallisation, possess the same crystalline form. 
There is, therefore, a correlation between analogy of 
composition and identity of crystalline form, and it is 
this which constitutes the discovery. After having esta- 
blished this correlation for salts of the same base formed 
by two different acids, Mitscherlich observed it again in 
analogous salts formed by the same acids united with 
different bases. Thus potash and ammonia on the one 
hand, and baryta, strontia, and oxide of lead on the 
other, form, with the same acids, salts analogous in 
composition and identical in crystalline form. The 
same identity of forms is found in the carbonates of 
lime, iron, zinc, and manganese. Mitscherlich was 
continually adding to his first examples. He showed 
the identity of the forms of certain sulphates of the 
magnesian group, which crystallise with the same 
quantity of water, such as the orthorhombic sulphates 
of magnesia, zinc, and nickel, and the clinorhombic 
sulphates of iron and cobalt; and the insignificant errors 


MITSCHERLICH—ISOMORPHISM. 57 


which crept into these early experiments had at least 
the advantage of strengthening his conviction. Though 
he was wrong in affirming that the different forms of 
sulphate of iron and sulphate of zinc are due to a dif- 
ference in the quantities of water of crystallisation, he 
shows that in magnetite and gahnite, which belong to the 
group of the spinels, the ferrous and zincic oxides form 
isomorphous combinations with ferric oxide.. He pre- 
pared iron alum, and showed its isomorphism with 
ordinary alum, &c. 

The first definition of the law of isomorphism—that 
acids of analogous composition and bases of analogous 
composition, the former with the same base, the latter 
with the same acid, form salts of identical crystalline 
form—was not absolutely correct. Mitscherlich himself 
recognised this subsequently. His discovery of di- 
morphism revealed the fact that the same substance can 
erystallise in two different forms. It must, therefore, 
be also possible for two ‘substances of different nature, 
but of analogous composition, to crystallise in two dif- 
ferent forms. . 

Facts of this nature, which are exceptions to the 
law, may be regarded as cases of dimorphism. Mit- 
scherlich also observed that substances which are really 
isomorphous, which combine and replace each other 
in the same crystal in every proportion, do not always 
present a perfect identity of form in isolated crystals, 
the number and value of the faces and angles being — 
liable to slight variations, though the general form of 
the crystal remains unchanged. 

Such is the discovery of Mitscherlich, and we must 


58 THE ATOMIC THEORY. 


now describe the influence which it has exercised upon 
the development of the atomic theory. 

Mitscherlich himself admitted, in his first memoir, 
that the similarity of properties in compounds of 
analogous composition and identical form could scarcely 
be attributed to identity of crystallisation, but that the 
explanation must be sought for in a primary and 
mysterious cause, to which must be referred, on the 
one hand, the fact of combination by fixed ‘ volumes’ 
(or atoms), and on the other the resemblance of crystals. 
This primary cause is the atomic structure of the 
bodies. A similar atomic constitution not only deter- | 
mines the analogy of. chemical properties, but also 
the similarity of physical forms. Mitscherlich thus 
declares himself a supporter of the ideas of Berzelius 
upon the constitution of bodies. Following the example 
of the latter, he expresses it first in ‘ volumes’ and 
afterwards in atoms.. The memoir published in the 
‘ Annales de Chimie et de Physique’ of 1821 bears the 
following significant title: Upon the identity of erys- 
talline form wm various substances, and the relation 
between this form and the number of elementary 
atoms in the crystals. We have already mentioned the 
restrictions which Mitscherlich was obliged to place 
upon his original idea. In a work presented to the 
Stockholm Academy in 1821 he endeavours to give 
them precision. He there proposes the following 
questions :—Do compounds formed by different elements 
with the same number of atoms of one or several other 
elements possess the same crystalline form? Is identity 
of crystalline form determined by the nwmber of atoms 


ISOMORPHISM. 59 


alone, and is this form independent of the chemical 
nature of the elements? 

The replies to these questions are not as absolute 
as the principle. For the form to remain unchanged 
in analogous compounds, the elements which replace 
each other must be mutually isomorphous, as phos- 
phorus and arsenic, or barium, strontium, and lead. 
Having further observed that certain salts, such as the 
acid phosphate of soda, can crystallise in different 
forms, although in the two cases the composition is 
identical, he attributes this dimorphism to a different 
arrangement of atoms. Thus the idea that chemical com- 
pounds are formed of atoms, and that the number and 
arrangement of these atoms exercise an influence upon 
the physical form of crystals, had evidently made an 
impression upon him; and the idea is natural, although 
it is founded upon a comparison some terms of which 
are wanting. A crystal may be compared to an edifice 
of definite form. We see its production, growth, and 
modification. Is it not natural to suppose that this 
form is due to the accumulation and arrangement of 
materials, which we call atoms? We are doubtless 
using figurative language when we compare the mole- 
cular edifice, and its construction from these invisible 
materials, to a monument of human architecture, which 
rises piece by piece before oureyes. But the necessities 
of the case are so perfectly answered by this repre- 
sentation that it has passed at once into the language 
of our general explanations and demonstrations. 

However this may be, the atomic theory evidently 
exercised an influence upon the conception of Mitscher- 


60 THE ATOMIC THEORY. 


lich and upon the manner in which. he stated his 
discovery. The same number of elementary atoms, he 
said, combined in the same manner, produce the same 
erystalline form, and this form is independent of the 
chemical nature of the atoms and determined solely by 
their number and arrangement. In spite of necessary 
restrictions and established exceptions, so great a law 
could not but act as a solid support to the atomic 
hypothesis, which had contributed such precise and 
simple terms for the enunciation of that law. 


Hays 


But this is not all. Mitscherlich’s discovery was 
the cause of important changes introduced by Berzelius 
into the system of atomic weights which he had esta- 
blished in 1813, and into the notation of which they are 
the origin. 

He had previously fixed the atomic weight of 
chromium and iron by attributing to chromic acid the 
composition CrO, and to ferric oxide the composition 
FeO, He now halves the atomic weight of chromium, 
attributing to chromic acid the formula CrO,, which 
makes it agree with anhydrous sulphuric acid, SO,. 
Chromium oxide now assumes the composition Cr,O,, 
and, on account of the isomorphism recognised between 
chromium oxide and ferric oxide (chrome alum and 
iron alum), the latter oxide becomes Fe,OQ, and ferrous 
oxide FeO. Thus was Berzelius forced to abandon an 
opinion which he had long entertained, one which 
Dalton, moreover, had never admitted—namely, that a 


BERZELIUS'’S SYSTEM OF ATOMIC WEIGHTS. 61 


binary compound (that is to say, a combination of two 
elements) must always contain a single atom of one or 
the other element. The existence of sesquioxides, 
R,O,, was finally admitted. 

But the halving of the atomic weights of chromium 
and iron occasioned other changes. 

The chemical analogies and isomorphism of ferrous 
oxide, FeO, with lime, magnesia, and oxide of zinc made 
it necessary to attribute to these oxides, and to the 
strong bases in general, the composition of protoxides, 
RO, and consequently to halve the atomic weights of 
a great number of metals, as indeed had already been 
done by Wollaston, Dulong and Petit (see p. 53). 
The old formule of the sulphates of iron and zinc— 


FeO,, 280, + 14H,0; ZnO,, 280, + 14H,O— 
became, therefore— 
- FeO, $0,+7H,0; ZnO, SO, +7H,0. 


These lower atomic weights agreed, moreover, 
with the law of specific heats. Berzelius draws atten- 
tion to this fact, and in future, in the determination 
of atomic weights, follows three principles which 
mutually support each other. 

1. The law of volumes. He steadily maintains the 
propositions which he had previously stated—namely, 
that equal volumes of simple gases contain an equal 
number of atoms. This proposition was soon to be 
refuted by the experiments of Dumas and Mitscherlich. 

2. The law of Dulong and Petit. This law is sub- 
ject, it is true, to some exceptions, but is of great as- 
sistance in certain cases, where it enables us to control 


62 THE ATOMIC THEORY. 


other determinations. The experiments of Regnault 
diminished the number of these exceptions, but it is 
only very recent investigations which have caused their 
final disappearance. 

3. The law of isomorphism. We have seen in the 
preceding pages the assistance which Berzelius obtained 
from this law in the determination of atomic weights. 
We here give the list of atomic weights as given by 
the great Swedish chemist in 1826, and repeated by hin 
without alteration in 1835. 


Atomic Weights 
Symbols | referred to 


Atomic Weights 
referred to Hy- 


Oxygen as 100 drogen as 1 

{a ae ee gen ede ho 100 | 16-02 
Hydrogen ; H 6°2398 1 

Carbon . : | C 76°44 12-26 
Boron ; ; B 136°2 21°82 
Phosphorus . i). eee 196°14 31°44 
Sulphur : 8 201°17 32°24 
Selenium ; Se 494°58 79°26 
Iodine . : I 789°75 126°56 
Bromine ‘ Br 489°75 78°40 
Chlorine ‘ Cl 221°33 35°48 
Fluorine : F 116°9 18°74 
Nitrogen : N 88°52 14:18 
Potassium , K 489-92 78°52 
Sodium : Na 290:90 46°62 
Lithium F L 80°33 12°88 
Barium : Ba 856°88 137°32 
Strontium ; Sr 547°29 87:70 
Calcium ‘ ; Ca 256°02 41:04 
Magnesium . : Mg 158°35 25°38 
Yttrium : ¥ 402°51 64°50 
Glucinum ; Gl 331°26 53°08 
Aluminium . : Al 1 hp ES i, 27°44 
Thorium : Th 744-90 119°30 
Zirconium ; Zr 420-20 | © “6734 
Silicon . P Si 277°31 44°44 
Titanium * Ti 303°66 48°66 
Tantalum ; Ta 1153-72 18490 
Tungsten pAaseaal 118300 — | 189-60 


BERZELIUS’S SYSTEM OF ATOMIC .WEIGHTS. 63 


Atomic Weights Atomic Weights 


Symbols referred to referred to Hy- 
Oxygen as 100 drogen as 1 
Molybdenum 598°52 95°92 
Vanadium 856'89 137°32 
Chromium 351°82 56°38 
Uranium 2711°36 434°52 
Manganese 345°89 55°44 
Arsenic 47004 75°34 
Antimony 806°45 129°24 
Tellurium 801°76 128°50 
Bismuth 886°92 142°14 
Zine 403-23 64°62 
Cadmium 696°77 111°66 
Tin 735°29 117°84 
Lead . 1294:50 297°46 
Cobalt . 368°99 59°14 
Nickel . 369°68 59°24 
Copper . 395°71 63°42 
Mercury 1265°82 202°86 
Silver 1351-61 216°60 
Gold 1243-01 199-20 
Platinum 1233°50 197°70 
Palladium 665°90 LOG TZ 
Rhodium ., : 651°39 104:40 
Iridium 1233°50 197°68 


Osmium 1244°49 


198-44 


The atomic weights of Berzelius are referred to 
oxygen as 100. Dividing the numbers which express the 
atomic weights by 6°2398, the atomic weight of hydro- 
gen, we obtain the numbers given in the second column, 
which are referred to hydrogen taken as unity. The 
comparison of these numbers with those adopted at the 
present day, which will be presently in leads to two 
important remarks. 

In the first place, the system of atomic weights 
which met with the approval of Berzelius is very similar 
to that which is adopted at the present day. With the 
exception of a few modifications which have been added 


64 . THE ATOMIC THEORY. 


to it,! and which do not affect the ruling principles and 
general features of the whole, we only discover one 
important difference between the two systems. This 
difference arises from the atomic weights of the alkaline 
metals, and of silver, which are twice as great as those 
which chemical analogies and the law of Dulong and 
Petit have now forced us to adopt. We shall presently 
return to this point. Secondly, in examining these 
numbers of Berzelius, we are struck by their accuracy. 
Most of these numbers only differ in the decimals from 
those which we now adopt as true. Such is the result 
of the enormous amount of labour expended by the 
Swedish chemist upon the determination of the atomic 
weights. It is a lasting monument which he has raised 
to science and his own glory. 

Nevertheless Berzelius never succeeded in persuad- 
ing all chemists to adopt his system of atomic weights. 
Dissentient voices are always to be heard. Gay-Lussac 
and Wollaston, following the example of Dalton and 
Thomson, adhered to the atomic weights derived solely 
from the consideration of the equivalent quantities 
which enter into combination. Gmelin adopted the 
same ideas in the several editions of his great work, 
and contributed greatly in the course of time towards 
the introduction of the equivalent notation. 

Berzelius made a concession upon one point to all 
these opponents. He introduced the idea of double 
atoms and applied it to certain gases, such as hydro- 
gen, nitrogen, chlorine, bromine, and iodine, the atomic 
weights of which were only half those admitted by 


1 Among others the atomic weights of uranium, silicon, &c. 


BERZELIUS’S SYSTEM OF ATOMIC WEIGHTS. 65 


other chemists. These double atoms were supposed to 
enter into combination in pairs, and every pair repre- 
sented precisely what others termed ‘the proportion’ or 
‘equivalent.’ Water was therefore composed of a 
double atom of hydrogen united to one atom of oxygen, 
and this combination was represented by the symbol 
HO. Hydrochloric acid and ammonia were formed, 
the first of one double atom of hydrogen united to one 
double atom of chlorine, the second of one double 
of nitrogen united to three double atoms of hydrogen. 
The formule HO, HE€l, HN, were really equivalent to 
the formule H,O, H,Cl,, H,N,, but remind us of the 
notation HO, HCl, H,;N, employed by Gmelin and 
others. It was, in fact,astep backwards. In admitting 
double atoms Berzelius unnecessarily doubled a number 
of formule ; and if itis true that H,O, H,Cl,, represent, 
from acertain point of view, equivalent quantities of 
water and hydrochloric acid, it is equally true that 
these formulz do not represent true molecular magni- 
tudes. Gerhardt subsequently showed that if a molecule 
of water, occupying two volumes of vapour, is repre-~ 
sented by the formula HO, a molecule of hydrochloric 
acid occupying two volumes of vapour should be repre- 
sented by the formula HCl, and a molecule of ammonia 
by H,N. It is true that the formula H,Cl, corresponds 
to the formula PbCl,, ZnCl,, CaCl,, and KCl,, by which 
Berzelius represented the chlorides of lead, zine, calcium, 
and potassium. But we now know that the molecules 
of all these chlorides are not, strictly speaking, equiva- 
lent, and that if the three first are true the fourth must 
be halved. The law of specific heats forces us, in fact, 
4 


66 THE ATOMIC THEORY. 


to halve the atomic weight of potassium, and conse- 
quently to represent its chloride by the formula KCl, 
which answers to HCl. The latter formula represents 
two volumes of vapour, as do the formule of water, H,O, 
and of ammonia, H,N. 

All these inaccuracies which we have pointed out in 
Berzelius’s system of atomic weights and notations arose 
from an erroneous conception of the law of volumes. 
Instead of regarding as equidistant, and equally distri- 
buted in equal volumes of gases or vapours, the particles 
of the second order, or the molecules of simple and com- 
pound bodies, as Avogadro and Ampére had done, and 
later Gerhardt, Berzelius only considered the primordial 
atoms of certain simple gases, holding that they alone, 
and not the ‘compound atoms,’ are’ distributed in 
equal numbers in equal volumes. We know now that 
this is an erroneous idea, and that the hypothesis of 
Avogadro and Ampére, long forgotten, but restored 
to its due place of honour by Gerhardt, applies to the 
single molecules or particles of the second order, which, 
whether simple or compound, constitute the ponder- 
able matter of gases and vapours. 


CHAPTER IV. 


SYSTEM OF CHEMICAL EQUIVALENTS—EQUIVALENT NOTATION. 
iB 


Tue interpretation which Berzelius had given of the 
law of volumes formed, as we have seen in the pre- 
ceding pages, one of the foundations of his system of 
atomic weights and of his notation. This foundation 
was destroyed by the researches of Dumas, and subse- 
quently of Mitscherlich, upon vapour densities com- 
menced in 1827. Dumas noticed that the vapour 
density of mercury is sensibly equal to 100, hydrogen 
being taken as unity. The vapour densities of mer- 
cury and of oxygen are as 100:16 or as 50:8. -If 
the atomic weights were proportional to the densities, 
8 of oxygen should combine with 50 of mercury to 
form mercuric oxide. This is not the case; mercuric 
oxide is composed of 8 of oxygen and 100 of mercury, 
and it is the latter number which Berzelius had adopted 
for the atomic weight of mercury. If equal volumes of 
oxygen and of mercury vapour contain the same num- 
ber of atoms, their densities should be in the ratio of 8 to 
100, or, in other words, the density of mercury vapour 


68 THE ATOMIC THEORY. 


is only half what it should be. We have here evidently 
a well-marked exception, or, better, a manifest contra- 
diction between the facts and. the principle admitted 
by Berzelius. Other exceptions may be mentioned. 
The vapour densities of sulphur and phosphorus deter- 
mined by Dumas in 1832 were found to be, in the first 
case, three times as great, and in the second twice as 
great, as those indicated by theory. Chemical con- 
siderations have caused a composition, expressed by the 
formule H,S and SQ,, to be attributed to sulphuretted 
hydrogen and sulphuric anhydride. From these formulz 
the ratio between the atomic weights of sulphur, 
oxygen, and hydrogen is expressed by the numbers 
32:1 :16, and the densities should be in the same 
ratio. Now, the vapour density of sulphur taken 
at about 560° is 96, hydrogen being taken as unity. 
From this density a quantity weighing 32 in the mole- 
cule of sulphuretted hydrogen would not represent an 
atom of sulphur, but 3 of an atom, and the formula of 
sulphuretted hydrogen, expressed in conformity with 
the law of volumes, would be H,S3, which is inadmis- 
sible. 

From the formulz PH, and P,O,, adopted for phos- 
phoretted hydrogen and phosphoric anhydride respec- 
tively, the relation between the atomic weights of 
phosphorus, hydrogen, and oxygen should be expressed 
_ by the numbers 31:1:16. Now, the vapour density 
of phosphorus is equal to 2x 31=62. If, therefore, the 
density of sulphur vapour is three times greater than 
that indicated by theory, that of phosphorus is twice as 
great. The case is the same with that of arsenic, from 


BERZELIUS'S PRINCIPLE OF NOTATION. 69 


an experiment of Mitscherlich, who also confirmed, in 
1833, the results obtained by Dumas upon the vapour 
of mercury, sulphur, and phosphorus. 

We here, therefore, meet with a serious difficulty. 
For its solution two courses are open to us: we must 
either maintain the principle of the equality of the 
number of atoms in equal volumes of gases or vapours, 
and determine to assign to mercury, sulphur, phosphorus, 
and arsenic atomic weights which shall conform to the 
vapour densities, although they are less probable, and 
consequently to give their compounds the formule Hg,O, 
H,S3, P5H, ; or else it will become necessary to sacri- 
fice the principle under discussion, in order to enable 
us to adopt the atomic weights, HgO, indicated by 
chemical analogies and the law of specific heats. The 
atomic weights of mercury, sulphur, phosphorus, and 
arsenic being, therefore, 200, 32, 31, 75, referred to 
hydrogen as unity, the preceding formulze become 
HgO, H,S, PH, and AsH,. 

It is the latter course which chemists have adopted, 
since they were properly unwilling to neglect more 
evident analogies. But the adoption of these atomic 
weights involves the following consequences :— 

1. The vapour of mercury, the density of which is 
only half that required by the atomic weight assigned 
to mercury, evidently contains half the number of 
atoms contained in an equal volume of hydrogen. 

2. The vapour of sulphur, which at 500° is three 
times as dense as it should be from the atomic weight 
assigned to sulphur, contains, at this temperature, three 
times the number of atoms contained in an equal 
volume of hydrogen. 


70 THE ATOMIC THEORY. 


3. The vapours of phosphorus and arsenic, which 
are twice as dense as they should be from their atomic 
weights, evidently contain twice as many atoms as an 
equal volume of hydrogen. 

The atomic constitution of gases or of elementary 
vapours is not, therefore, always the same, as Berzelius 
for a long time supposed. If we compare gases or ele- 
mentary vapours, as far as concerns the number of atoms 
which they contain, to the vapour of mercury, which 
contains the least, we shall have the result that, if 
mercury vapour contains in a certain volume one atom, 
hydrogen, oxygen, nitrogen, chlorine, bromine, and 
iodine contain 2, phosphorus and arsenic contain 4, 
while sulphur at 500° contains 6. The relations be- 
tween the number of atoms contained in equal volumes © 
of gases or of vapours may be obtained by dividing the 
density of the gas or vapour by the corresponding 
atomic weight. We shall thus obtain the following 
results :-— 

Densities 
divided by 
Atomic 


Atomic Weights. Number of 


: Atcms in 
Weights Number of ; 
Atoms in 2 Volumes 


Unit of 
Volume 


Densities 
referred to 
Hydrogen 


Or 


Mercury 
Hydrogen 
Oxygen 
Nitrogen 
Chlorine 
Bromine 
Iodine . 
Phosphorus . 
Arsenic , 
Sulphur at 500? 


NGI Nl ll ell ll ll oll ek) 
SH EH DO ND DOD ww Ne 


roe) 


BERZELIUS'S PRINCIPLE OF NOTATION. 71 


We have, therefore, to distinguish monatomic, 
diatomic, tetratomic, and hexatomic gases. Gmelin has 
already introduced into science a similar distinction, 
which has now become so important. At p. 54 of the 
first volume of the fourth edition of his treatise he 
gives a table analogous to the preceding, with some dif- 
ferences due to the different. atomic weights adopted. 
Those used in our table are those of Berzelius (p. 62), 
which are now adopted for the respective elements. 

With Gmelin and other chemists who soon followed 
his example it was different. As we have already 
remarked, the former maintained the proportional 
numbers which he designated in the first editions of his 
classical treatise by the erroneous term of * Mischungsge- 
wichte,’' and which he referred to hydrogen as unity, 
following the example of Dalton. In the fourth edition 
of his work he returns to the term atomic weights, but 
the numbers thus designated were identical with the 
proportional numbers or equivalents. 


iN 


The system of chemical equivalents and the notation 
derived from them gradually prevailed over the system 
of atomic weights and the notation of Berzelius, and 
are still preferred by some French chemists. It will 
therefore be useful to explain the principles upon which 
this equivalent notation rests, and particularly the 
arguments used by Gmelin against Berzelius in the 
question which forms the chief point of the discussion— 


1 Literally ‘mixing weights,’ instead of ‘combining weights. 


fe THE ATOMIC THEORY. 


viz. the atomic weights of hydrogen, nitrogen, phos- 
phorus, arsenic, chlorine, bromine, and iodine, which 
are half the proportional numbers or equivalents. 

1. The atomic weights are deduced from the densi- 
ties of the gases, and are founded upon the hypothesis 
that these gases contain an equal number of atoms in 
equal volumes. Now, this hypothesis is contradicted 
by experiment, as far as concerns the vapours of sulphur, 
phosphorus, arsenic, and mercury. Gmelin also re- 
marked that certain gases — hydrochloric acid, for 
example—contain in equal volumes only half as many 
atoms as chlorine and hydrogen.! 

There is, therefore, no reason for the adoption of the 
halved atomic weights of Berzelius, and his double 
atoms are the true atoms—that is to say, the equiva- 
lents. 

2. The small atoms of which he speaks never enter 
singly into any combination. Neither do they ever 
enter into combination in uneven numbers, such as 3, 
5, 7, &c., but always in even numbers, such as 2, 4, 6. 
Thus water contains H,O, and hydrochloric acid H,Cl,, 
and ammonia H,N,. The atomic weights of hydrogen, 
chlorine, and nitrogen ought, therefore, to be doubled, 
so as to give the above compounds the simple formule 
HO, HCl, and H.N. 

3. We should admit that heterogeneous atoms 
unite in the simplest proportions, and the value of the 
atomic weights ought to be doubled-in order to repre~ 
sent these simple relations. ‘Thus, when a metal only 
combines with oxygen in a single proportion, it should 


1 HCl was then called an atom of hydrochloric acid. 


THE OBJECTIONS BROUGHT FORWARD BY GMELIN. 73 


be supposed that the combination takes place atom with 
atom, unless isomorphism points to the contrary; and 
when a metal forms several combinations with oxygen, 
the strongest base ought to be supposed to be a com- 
bination of one atom of metal with one atom of oxy- 
gen. 

4. The sum of all the atomic weights of an acid 
should represent the weight of the acid which saturates 
a quantity of base containing one atom of oxygen. 
Thus 40 parts of sulphuric acid saturate 111°8 parts of 
oxide of lead, which contain 8 of oxygen (one equiva- 
lent) and 103°8 of lead (one equivalent). These 40 
parts consequently represent the sum of the equivalents 
of oxygen (24=3 x 8) and of sulphur (16); 16 is there- 
fore the equivalent of sulphur, the formula of sulphuric 
acid being SO,. The question of equivalence is here 
very clearly put, almost in the terms used by Dumas in 
1828 in the first volume of his celebrated treatise on 
chemistry applied to the arts. 

5. The same formule ought to be assigned to iso- 
morphous compounds as well as to compounds of the 
same order formed by simple and similar bodies, such 
as cobalt and nickel. 

We will briefly examine into the value of these argu- 
ments,’ reserving the development of some of the points 
here mentioned for the following chapter. 

1, Gmelin justly observed that there are exceptions 
to the principle laid down by Berzelius of the equality 
of the number of atoms in equal volumes of a gas or a 


No objection can be made to the last principle (No. 5), which 
is followed by all notations. 


74 THE ATOMIC THEORY. 


vapour, and that consequently the weights of equal 
volumes do not always represent the atomic weights. 
Thisisadmitted. But when he remarks that one volume 
of hydrochloric acid gas only contains half the number of 
atoms contained in one volume of chlorine or of hydrogen, 
he evidently confuses atoms with molecules. It is now 
admitted that equal volumes of these gases contain the 
same number of molecules, and we may remark that, in 
the present case, they also contain the same number of 
atoms, as is shown by the following formule :— 


H,=2 vol. Cl, =2 vol. HCl=2 vol. 
1 molecule of 1 molecule of 1 molecule of hydro- 
hydrogen. chlorine. chloric acid. 


2. It is not correct to say that the small atoms of 
Berzelius do not enter into combination in uneven 
numbers. Ifa molecule of water is represented by the 
formula H,O=2 vol., a molecule of hydrochloric acid is 
represented by the formula HC]l=2 vol., and a molecule 
of ammonia by the formula NH,. The double formule 
of Berzelius, H,Cl, and N,H,, did not represent the true 
molecular masses; they were double the true number 
and ought to be halved, as was first proposed by 
Gerhardt. The small atoms of Berzelius, H=6-24, 
Cl= 221°3, N= 88:5, therefore represent the true atomic 
weights of these elements, referred to oxygen as 100, and 
the ratio between these numbers is identical with 
the ratio between the numbers 1 : 35:5 : 16, which 
are now accepted as the atomic weights of these ele- 
ments. 

3. The statement is correct that heterogeneous 
atoms generally unite together in very simple propor- 


INCONSISTENCIES IN THE EQUIVALENT NOTATION. 75 


tions. This fact becomes evident if we allow ourselves 
to be guided in determining atomic weights and in 
constructing formule, not only by chemical considera- 
tions, but also by the great physical laws which have 
been described—namely, the law of volumes, of specific 
heats, and of isomorphism. Purely chemical considera- 
tions might lead us into error. Thus it is not correct 
to say that the strong bases ought always ‘to contain 
one atom of metal and one atom of oxygen. Lime, 
baryta, strontia, cupric oxide, mercuric oxide, &c., con- 
tain, it is true, 1 atom of metal and 1 atom of oxygen; 
but oxide of silver, which is a strong base, contains 2 
atoms of metal for 1 of oxygen, the atomic weight of 
silver being determined by the law of specific heats. 
As far as concerns oxide of silver, therefore, we make a 
mistake if we invoke analogy in order to connect it 
with the preceding oxides in respect to its atomic 
constitution. 

4. The principle of equivalence made use of by 
Dalton, Wollaston, Gay-Lussac, and Gmelin for the 
determination of equivalents (which Dalton and Gmelin 
called atomic weights) would be admirable if it could 
be applied rigorously either to elements or to com- 
pounds. 

But we now know that all atoms are not equivalent, 
and that the case is the same with molecules and with 
the reactions to which they give rise. ; 

Atoms differ in their combining or substituting value 
—in their valency, as it is called—molecules in their 
state of condensation and their degree of saturation, and 
reactions in the greater or less extent of their com- 


76 THE ATOMIC THEORY. 


plexity. As we have remarked above concerning 
oxides, it is impossible to cast all this in the same 
mould. 

To return to the exact point of the discussion, it is 
impossible to consider a molecule of nitric acid ‘and of 
phosphoric acid as equivalent; and if, in conformity 
with the rule laid down by Gmelin, 14 is the equivalent 
of nitrogen because nitrate of silver contains 14 parts 
of nitrogen for 108 of silver, 10°5 should be the equiva- 
lent of phosphorus, for it is the weight of phosphorus 
contained in a quantity of phosphate of silver contain- 
ing 108 parts of silver. Now, all chemists admit that 
the equivalent of phosphorus is 31:4; but then we 
must no longer consider a molecule of nitric acid as 
equivalent to a molecule of phosphorie acid, for if the 
former saturates a quantity of oxide of silver containing 
1 atom of silver, the latter saturates a quantity of oxide 
of silver containing 3 atoms. In fact, the discovery of 
polybasic acids proved a serious difficulty to the theory 
of equivalence ; it showed that chemical molecules are 
not equivalent, as was shown for atoms by the law of 
volumes. Moreover, Gmelin felt that he had met with 
a difficulty, for he mentions polybasic acids as forming 
an exception to the theory of equivalence. It is some- 
times said—I do not know for what reason—-that 
‘exceptions prove the rule; in the present case they 
have become so numerous and so striking that they 
have overthrown it. The discovery of polybasic acids 
has, in fact, been supplemented by other discoveries, 
and they have completely modified the old ideas upon 
the equivalence of molecules and of reactions. But 


INCONSISTENCIES IN THE EQUIVALENT NOTATION. 77 


this is not the proper place to develope this point, and 
we will merely add a remark which seems important. 

Dalton and Gay-Lussac alone applied true principles 
to the determination of equivalents. Dalton attributed 
to phosphorus the atomic weight 10°3; it represents 
the quantity of phosphorus which combines with 1 part 
of hydrogen: to carbon the atomic weight 4°3 (instead 
of 6); it represents the quantity of carbon which unites 
with 1 of hydrogen to form bicarburetted hydrogen. 
Gay-Lussac started from another point of view. Con- 
sidering ordinary phosphate of soda as neutral, he 
admitted in this salt the presence of one equivalent of 
base and consequently one equivalent of sodium. He 
therefore expressed its composition by the formula 
PO,3.Na0 + Aq,'! and attributed to phosphorus the 
proportional number 15:7. The quantity of neutral 
phosphate of soda which is proportional or equivalent 
to a molecule of nitrate of soda, NO,.NaO, or of silver, 
NO,.AgO, ought, in fact, only to contain 1 atom of 
metal, like the latter. 

Applying the same principles in other cases, he 
wrote ferrous oxide FeO and ferric oxide Fe20. 

Ferrous sulphate, SO,.FeO, was strictly equivalent 
to ferric sulphate, SO,.Feg0. 

Berzelius, on the contrary, who had at last decided 


1 P=15'7; O=8. At this time no account was taken of basic 
water. Gay-Lussac therefore involuntarily.committed an error in 
the determination of the equivalent of phosphoric acid. In fact, 
the quantities of phosphate of soda and of nitrate of silver which 
enter into reaction, and which are strictly equivalent, are 
4(PO,Na,H) and NO,Ag, and the quantity of phosphorus in 
4(PO,Na,H) is 10°5. This is the number of Dalton. 


78 THE ATOMIC THEORY. 


to admit the existence of sesquioxides, proved that they 
unite with 3 atoms (molecules) of acid. He con- 
sequently represented ferrous and ferric sulphates by 
the formule SO,.FeO and 3S0,.Fe,0,. Is it not 
evident that he was less consistent than Gay-Lussac, 
and that these formule do not represent equivalent 
quantities? It is only a strange abuse of language, not 
to say a logical error, to consider as equivalent a mole- 
cule of ferric oxide, which saturates 3 molecules of 
sulphuric acid, and a molecule of ferrous oxide, which 
only saturates 1 molecule. Formule analogous to those 
of the sulphates of the sesquioxides, such as those of the 
phosphates and of several other compounds, which are 
now distinguished by the name polyatomic, reveal, there- 
fore, serious inconsistencies in the equivalent notation, 
and we must choose between such inconsistencies and 
the graver inconvenience of misrepresenting reactions 
by referring them to strictly equivalent proportions. 
This point will be developed in the following chapter. 

The preceding discussion renders it sufficiently 
evident that the system of chemical equivalents, and of 
the notation derived from them, introduced by Dalton, 
Wollaston, Davy, Gay-Lussac, and Gmelin, were based 
upon too narrow a foundation for the enlarged edifice of 
chemistry. Our present system of atomic weights and 
our notation rest upon a wider foundation. Their 
establishment has required the numerous efforts which 
have been perseveringly maintained for a period of 
thirty years. 


CHAPTER V. 


PRESENT SYSTEM OF ATOMIC WEIGHTS. 


GERHARDT AND LAURENT—CANNIZZARO, 


ye 


Tue equivalent notation of the English chemists and 
of Gay-Lussac, which was adopted by Liebig and 
defended by Gmelin in 1843, had, at the period of 
which we are speaking, gained the almost unanimous 
approval of chemists; they were struck with the excep- 
tions presented by the law of volumes as it was then 
interpreted, by the useless complication which the con- 
ception of the double atoms of Berzelius had introduced 
into a large number of formulz, and they were satisfied 
with the more simple expressions which the notion of 
equivalents offered for chemical reactions and com- 
binations. The law of volumes was entirely sacrificed. 
The equivalents of hydrogen, nitrogen, chlorine, &c., 
corresponded to two volumes, whilst that of oxygen 
only constituted one. The formule of water, HO, of 
sulphuretted hydrogen, HS, of protoxide of nitrogen, 


80 THE ATOMIC THEORY. 


NO, expressed two volumes; those of hydrochloric acid, 
HCl, of ammonia, NH,, of phosphoretted hydrogen, 
PH,, &c., represented four. 

Gerhardt was the first to draw attention to these 
errors, and to the necessity of considering as equivalents 
quantities of water, ammonia, hydrochloric acid, &c., 
corresponding to equal volumes. Regarding water, H,O, 
as formed of two atoms or volumes of hydrogen and as 
occupying 2 volumes, if one atom of hydrogen occupies 
one volume, he compares it to hydrochloric acid, HCl, 
formed of one atom or volume of hydrogen and of one 
atom or volume of chlorine, and occupying 2 volumes; 
to ammonia, NH,, formed of one atom (volume) of 
nitrogen and of 3 atoms (volumes) of hydrogen, and 
occupying 2 volumes. In the same manner the for- 
mule N,O, NO, CO, CO,, CH,, C,H,, which correspond to 
2 volumes, represent molecules (Gerhardt still used the 
term equivalents) of protoxide of nitrogen, dioxide of 
nitrogen, carbonic oxide, carbonic acid, and of marsh 
gas and olefiant gas. The atomic weights on which the 
preceding formulze are founded are the same as those 
of ‘Berzelius, Le. O=100; H=6°25, N=88," C= 75, 
But the formule of hydrochloric acid, H,Cl,, of 
ammonia, N,H,, of marsh gas, C,H,, of olefiant gas, C,H,, 
which Berzelius had employed were halved and made 
to represent 2 volumes. Here lies the true progress. 

It will be interesting to recall the considerations 
which led Gerhardt to propose ue reform in the nota- 
tion of Berzelius. for 

Regarding a molecule of water as formed of 2 atoms 
of hydrogen and 1 atom of oxygen, and carbonic acid as 


GERHARDT’S NOTATION. 81 


containing 1 atom of carbon and 2 atoms of oxygen, he 
was struck, in the attentive study of the reactions of 
organic chemistry, by the fact that in none of these 
reactions, represented by the formulz and equations of 
Berzelius then in use, were quantities of water and car- 
bonie acid corresponding to H,O and CO, set free, but 
that the quantities formed were never less than those 
corresponding to the formulze H,O, and C,0,. 

We may therefore conclude, he says, that an error 
has been committed in the construction of organic 
formule, for it would be strange if no redaction should 
give rise to the formation of a single molecule of water 
or a single molecule of carbonic acid. This is the 
error: organic formulze are twice as great as they should 
be, and must be halved, as well as the atomic weights 
of metals. These two facts are correlative, and it was 
precisely those high atomic weights attributed by Ber- 
zelius to the metals which gave to organic compounds 
formule double what they should be. Thus amongst 
the organic combinations with which we are most 
fully acquainted we must reckon the acids ; their mole- 
cular magnitude is determined by their capacity of 
saturation, and we admit that a molecule of acid 
saturates a molecule of basic oxide—that is to say, a 
quantity of base containing one atom of metal. Thus, 
for example, the formula of acetic acid is constructed by 
combining it with oxide of silver and analysing the 
acetate of silver. The composition of this salt, contain- 
ing one atom of silver, is represented by the formula 
C,H,AgO,, derived from the atomic weights C=75, 
H=6°25, O=100, Ag=1351°6, which are those of Berze- 


82 THE ATOMIC THEORY. 


lius. But upon halving the atomic weight of silver we 
obtain Ag=675°'8; the preceding formula will become 
C,H,Ag,O,; and there is no reason why we should not 
halve this, for we must admit that the monobasic acetic 
acid only contains in its salts a single atom of metal. 
The true formula of acetate of silver and acetic acid 
are therefore C,H,AgO, and C,H,0,. 

But why must we halve the atomic weights of metals 
in this manner? In order that their oxides may be 
comparable to water. If the latter is formed of 2 atoms 
of hydrogen, we may reasonably attribute to protoxides 
a similar composition, and represent them by the 
formula M,O instead of MO. Oxide of potassium and 
oxide of silver being, therefore, K,O and Ag,O, the 
atomic weights of potassium and silver will be 245 and 
687°5 '—that is to say, the half of those attributed to 
them by Berzelius, the atomic weights of hydrogen 
and oxygen being 6°25 and 100. Applying the same 
considerations to the other protoxides, Gerhardt also 
halved the atomic weights of the metals which they 
contain. We shall presently see that in this he went 
too far; but this reasoning was perfectly correct as far 
as it concerned acetate of silver, and nothing could be 
more legitimate than the halving of the formula of 
acetic acid, the unnecessary complication of which he 
was the first to show. And this change demanded 
others. It is clear that the several monobasic acids, the 
alcohols, ethers, amides, &c., must be represented by 
formule which harmonise with that of acetic acid. 


1 The number 687°5 is deduced from a determination of Erdmann 
and Marchand (Précis de Chimie organique, t. i. p. 54). 


GERHARDT’S NOTATION. 83 


This led to an important reform in the notation of 
organic compounds, which reform extended even to in- 
organic compounds. JBerzelius had represented hydro- 
chloric acid by the formula H,Cl,, because 100 being 
the atomic weight of oxygen, this quantity of hydro- 
chloric acid was necessary to saturate a molecule of 
oxide of silver containing 1351°6 of silver and 100 of 
oxygen. The formula H,Cl, is therefore in harmony 
with the formule KCl,, AgCl,, PbCl,, which represent 
the composition of the protochlorides. But when the 
atomic weights of the metals were halved, it was con- 
sidered advisable to attribute to all these chlorides the 
more simple formule HCl, KCl, AgCl, PbCl. 

The reform which Gerhardt introduced into notation 
necessitated certain modifications in the existing ideas 
concerning the constitution of salts. It can now no 
longer be said that a molecule of acetate of silver contains 
a molecule of anhydrous acetic acid and a molecule of 
oxide of silver, or that hydrated acetic acid contains a 
molecule of anhydrous acid and a molecule of water. 
The double formule favoured these interpretations, while 
the simple formule cannot be divided in such a manner. 
Though C,H,Ag,O, might be decomposed into 
C,H,O,+Ag.0, and C,H,O, into C,H,O,+H,0, the 
formule C,H,AgO, and C,H,O, could not be divided so 
as to give anhydrous acid and oxide of silver, or anhy- 
drous acid and water. Yet the relations between acetic 
acid and acetate of silver are very simple, and correctly 
defined when we say that acetate of silver consists of 
acetic acid in which one atom of hydrogen is replaced by 
an atom of silver. The molecule of acetic acid may, 


84 THE ATOMIC THEORY. 


then, be regarded as a group of atoms in which an atom of 
hydrogen, which is called basic, can be replaced by an. 
atom of metal, in the same manner as, in a different 
class of facts, the three other atoms of hydrogen may 
be replaced by three atoms of chlorine. 

This is an important consequence of Gerhardt’s 
notation, to which I thought it well to draw attention 
in passing, for these ideas upon the nature of salts were 
opposed by their great author to the dualistic theory, 
and are in harmony with the proposition which was at 
that time supported by Dumas, Laurent, and the advo- 
cates of the substitution theory, which teaches that 
chemical combinations form a whole, a unit. ‘This was 
at that time—perhaps improperly—called the unitary 
system. 

But to return to the point under discussion: I have 
just mentioned Laurent, and we should notice the fact 
that he was the first adherent of Gerhardt’s system of 
atomic weights and notation. I think it will be also 
interesting to recall some of the ideas which he then 
published in connection with this notation. 

We admit that oxide of potassium is formed of 2 
atoms of potassium and | atom of oxygen, but caustic 
potash or potassium hydrate cannot be regarded as con- 
taining the elements of oxide of potassium + the 
elements of water. The molecule of potassium hydrate 
may be compared on the one hand to that of oxide of 
potassium, on the other to that of water, and is derived 
in a manner from the latter by the substitution of an 
atom of potassium for an atom of hydrogen. 

Thus water, potash, and anhydrous oxide of potassium 


LAURENT’S IDEAS. 85 


are compounds of the same order containing respec- 
tively a single atom of oxygen combined either with 
2 atoms of hydrogen, 2 atoms of potassium, or with 1 
atom of potassium and 1 atom of hydrogen. The 
metallic hydrates are, therefore, compounds of the same 
order as the oxide, and cannot be represented as con- 
taining an anhydrous oxide + water. But there are 
also organic hydrates and oxides; and if we admit, in 
aleohol and ether, the existence of an ethyl group, so 
termed by Berzelius, we shall observe the same relations 
between water, alcohol, and ether as those which exist 
between water, potash, and oxide of potassium. Alcohol 
becomes ethyl hydrate, and ether ethyl oxide. The 
following formule, in which Et represents the ethy! 
group O,H,, will show these analogies :— 


H,0O, water. _ H,O, water. 
KHO, potassium hydrate. EtHO, alcohol. 
K,0, potassium oxide. Et,0, ether. 


This grand generalisation was afterwards extended 
by Gerhardt, who had first discovered the acid chlorides 
and anhydrous monobasic acids, to the acids. Upon 
comparing Ac(l, chloride of acetyl, with EtCl, chloride 
of ethyl, and hydrochloric acid, the same kind of rela- 
tions were discovered between acetic acid and anhydrous 
acetic acid as those between alcohol and ether. The 
salts and ethers of acetic acid can, as Williamson has 
shown, be added tv this synoptic table, which formed 
the basis of the celebrated idea of considering hydro- 
chloric acid and water as types :— 


86 THE ATOMIC THEORY. 


HCl, hydrochloric actd. H,0, water. | 
EtCl, chloride of ethyl. AcHO, acetic acid. 
AcCl, chloride of acetyl. AcKO, acetate of potassium. 


KCl, chloride of potassium. AcEtO, acetic ether. 
Ac,O, acetic anhydride. 


It is from the new notation that these views, which 
embrace the discoveries of Williamson on etherification 
and those of Gerhardt on anhydrous acids, derive their 
simple and striking forms. The molecules of all the 
bodies just mentioned are comparable, under the con- 
dition that they are represented, in accordance with 
the principles developed by Gerhardt, by formule 
which represent the true molecular magnitudes. And 
it is important to remark that all these formule corre- 
spond, in the case of volatile compounds, to 2 volumes 
of vapour. ‘ We halve,’ he says, ‘ organic and mineral 
formule, so as to express their equivalent by 2 volumes.’ 
‘ Equivalent’ is used instead of ¢ molecule,’ and from the 
preceding proposition we conclude that equal numbers 
of the molecules of gaseous or volatile compounds are 
contained in equal volumes of gases or vapours. This 
is the law of Avogadro and Ampére, which reappears as 
a guiding star upon the horizon after a long eclipse. 
And yet we cannot say that Gerhardt, at this period at 
at: least, gave himself up entirely to its guidance. The 
considerations by which he was principally influenced 
were rather of a purely chemical character—those which 
we have alluded to above. They were correct, and 
were found to agree with an equally correct idea which 
had been forgotten. The distinction between two species 

of minute particles, moleculesand atoms,which Avogadro 


GERHARDT’S NOTATION. 87 


and Ampére had introduced without effect into science, 
and which Dumas had endeavoured to reproduce in his 
‘Chemical Philosophy,’ was probably mentally clear to 
Gerhardt, though as yet it had not appeared in his 
writings. The word ‘ equivalent ’ was sometimes synony- 
mous with the term ‘molecule, sometimes with ‘atom’ or 
‘volume.’ To quote his own words, ‘ Therefore,’ he says 
in p. 51 of his ‘ Précis, ‘ volumes, atoms, and equi- 
valents are synonymous in the case of simple bodies. 
It therefore follows that the densities of simple gases 
are proportional to thew equivalents. These pro- 
positions were not new, but they were inaccurate. 
These inaccuracies soon disappeared, and the distinction 
between molecules and atoms appeared clearly in the 
classic ‘ Traité de Chimie organique.’ 

Gerhardt’s system of atomic weights, which was 
immediately adopted by Laurent, gradually gained the 
approval of a great number of chemists. His works 
upon the theory of types, the discovery of the anhydrides 
and the chlorides of the monobasic fatty acids, gave 
him great authority, which profited him but little 
personally, but which will always be connected with 
his name. The simplicity of the new notation gave 
great clearness to the explanation of new facts and 
ideas. In England Williamson, Odling, Brodie, 
Frankland, Hofmann, Gladstone, Roscoe, and others 
successively adopted this notation. The new German 
school, which was then under the brilliant direction of 
Kekulé and Baeyer, adopted it at once, as also has been 
the case with the greater number of Russian and Italian 
chemists. In France Chancel has always made use of 


88 THE ATOMIC THEORY. 


it, and I myself did so in my memoir upon the glycols 
in 1858. | 


I. 


The commencement of the year which I have just 
mentioned was, however, marked by the introduction of 
an important change. Cannizzaro proposed once more to 
double the atomic weights of a great number of metals. 
We must now point out the facts and follow the course 
of ideas which have proved the reform introduced by 
the illustrious Italian to be legitimate and gained for it 
the almost unanimous approbation of chemists. 

Gerhardt’s atomic weights were not. true equivalents, 
and molecules which occupy the same volume in a 
gaseous state are not always compounds of the same degree 
or the same order ; for Gerhardt afterwards referred these 
compounds to three different types—the hydrogen or 
hydrochloric acid type, the water type, and the ammonia 
type. That the molecules of chemical compounds differ 
from each other in their type—that is to say, in their 
degree of complication or in their manner of condensa- 
tion (which, moreover, the discoveries of Gay-Lussac had 
already indicated)—and consequently that molecules 
belonging to different types are not strictly equivalent, 
was an idea which was gaining ground in science. 
Correctly speaking, it was not at that time perfectly new; 
since the admission of the existence of sesquioxides, such 
as alumina and ferric oxide, it had been found that their 
capacity of saturation was three times greater than that 
of the protoxides; the sesquioxides are polyacid bases. 


CANNIZZARO’S REFORM. 89 


On the other hand, Graham had already made the great 
discovery of polybasic acids. 

But other facts were soon added to the preceding, 
which introduced into science, if not the fact, at least 
the clearly defined notion of polyatomic compounds. I 
allude especially to the works of Berthelot upon gly- 
cerine, which produced such a number of important 
results, to which, I believe, I was the first to give their 
true interpretation in the order of ideas which we are 
now discussing. I must also mention Berthelot’s work 
upon the sugars and my own researches upon radicals 
and glycols, in which I endeavoured to define the part 
played by radicals in polyatomic compounds. These 
researches have introduced into science the idea that all 
chemical molecules are not mutually equivalent as far 
as their molecular complication is concerned, or, to use 
the phraseology of that time, ‘the degree of condensation 
affected in them by matter.’! In order to define the 
differences which they present in this relation, they were 
referred to more or less condensed types. Thus, to 
take a few examples, the constitution of nitric, sulphuric, 
phosphoric, acetic, and oxalic acids were represented by 
the following formulz :— 


H H H 
TYPE ef O+ Vi, | UTYPE 1} 9 TYPE Hs} 0; 
(NO x (S0,)" (PO)” 
iu 0 He, O, 1 O; 
Nitric acid. Sulphuric acid. Phosphoric acid. 
C,H,OY C,0,)” 
(CHLOY} (.09"! o, 
Acetic acid. Oxalic acid. 


1 Annales de Chimie et de Physique, 3e série, t. xliv. p. 308. 
5 


30 THE ATOMIC THEORY. 


Similar formule represented the constitution and 
the increasing complication of the molecules of potash 
and ferric hydrate, for example, and of those of alcohol, 
olycol, and glycerine :— 


H H H 
TYPE a O TYPE ae co TYPE His} 0, 
nL RY" 

co : eho 
Potash. Ferric hydrate. 
(C,H,y (C,H,)" (C,H, y” 
2 a oO 2 7 : 0, 3 “Hr, O, 
Alcohol. Glycol. Glycerine. 


These typical formuls had an advantage. They 
clearly indicated the fact that not only inorganic or 
organic radicals, but even simple bodies are capable of 
replacing 1, 2, or 3 atoms of hydrogen, and consequently 
differ in their substituting value. A distinction was 
therefore drawn between the monatomic, diatomic, and 
triatomic radicals. And as these radicals are in a 
manner nothing more than the representatives of the 
elements themselves, the distinction was extended to the 
latter. We shall presently develope this idea, that the 
power of combination or substitution with which radicals 
are endowed is essentially connected with that of the 
elements which they contain. But for a moment we 
must be contented with remarking that there is a gap 
between potash, which contains monatomic potassium, 
and ferric hydrate, which contains triatomic iron.! This 

1 The formula} O;, which has been proposed by Odling, 
clearly expresses this idea of triatomic iron. Fe’ here takes the 


place of H, in three molecules of water, Hi} Oz. 
8 


CANNIZZARO’S REFORM. 9] 


gap has been, thanks to Cannizzaro, in a great measure 
filled up. This eminent chemist has doubled the atomic 
weights of a great number of metals, to bring them into 
harmony with the law of Dulong and Petit and the law 
of Avogadro. ‘These metals have been regarded, there- 


fore, as diatomic. Their oxides have become RO. Their 


R 
hydrates, rt | 0, answered to the hydrates of the 
2 


diatomic radicals—for example, to ethylene hydrate or 
glycol—which is given in the preceding table. We must 
not forget the influence which the discoveries of organic 
chemistry, and the interpretation given to them, have 
exercised upon the general conceptions of chemistry, 
and even upon the progress of mineral chemistry. We 
shall, therefore, return to this point in treating of 
atomicity. 

We here give the list of atomic weights now adopted 
by the majority of chemists. And, in order that the 
changes which the new discoveries and the progress of 
the theory have successively introduced into the system 
of atomic weights may be appreciated, we have, in the 
following table, marked elements with a distinctive 
sign. ‘Those which are printed in ztalics represent the 
elements to which Berzelius and Gerhardt attributed the 
same atomic weights, which they now retain; those 
which are marked by an asterisk have retained Gerhardt’s 
atomic weights ; those, finally, which are marked by two 
asterisks are the metals whose atomic weights were 
halved by Gerhardt and doubled again by Cannizzaro, | 
these double numbers being, moreover, those of Ber- 
zelius (see p. 62) :— 


92 


THE ATOMIC THEORY. 


Hydrogen 


Aluminium*. 
Antimony* . 
Arsenic 
Barium** 
Bismuth** 
Boron* , 
Bromine 
Cadmium** , 
Cesium 
Calcium** 
Carbon 
Cerium 
Chlorine 
Chromium 
Cobalt** 
Copper** 
Didymium . 
Erbium 
Fluorine 
Gallium 
Glucinum 
Gold** 
Indium 
Todine . 
Iridium** 
Iron** 
Lanthanum 
Lead** 
Lithium* 
Magnesium** 
Manganese** 
Mercury** . 
Molybdenum** 


_Nickel** 


Niobium 
Nitrogen 
Osmium** 
Oxygen 5 


Symbols. 
H 


_—— 


Al 
Sb 
As 
Ba 
Bi 
B 

Br 
Cd 
Cs 
Ca 


Atomic 
Weights. 


1 

275 
122 

74:9 (75) 
137-2 


198°6 
15-96 (16) 


TABLE OF ATOMIC WEIGHTS. 93 


Atomic 

Symbols. Weights. 
Palladium** é ; oro 106°2 
Phosphorus ‘ : ; ae 31 
Platinum . . : ' * PG 196°7 
Potassium* : ‘ Tet ase 39°137 
Rhodium** : F 2 . Rh 104°2 
Rubidium* ? : . = lepiiy 85'2 
Ruthenium 3 : : =. 10 103°5 
Seleniwm . ‘ , : . Se 78 
Silicon* . i é : . Si 28 
Silver* . : : ; . Ag 108! 
Sodium* . : : s NS 23°043 
Strontium** . - : @.5r 87°2 
Sulphur . - F : eet 31°98 (32) 
Tantalum . f : ped be 182 
Tellurium / : - owa8 128 
Thallium . : F “ ee El 203°6 
Thorium . ; of eg bin) 233°9 
Tin** : p : : oe oil 117°8 
Titanium** . : ; pod byl 48 
Tungsten** é ; ee Wi 184 
Uranium . , : : 2 Us 120 
Vanadium : we ee inc 51:2 
Yttrium . : ’ : ae ¥ 89°6 
Zinc** ; : ; ee ris 64:9 
Zirconium ; : P me As Hye 


The limits which are imposed upon us by the 
character of this work make it impossible to mention 
the methods which have been employed in each parti- 
cular case for the determination of the atomic weights 
given in the preceding table. We must refer our readers 
for these details to the article upon Atomic Weights 
in the ‘ Dictionnaire de Chimie pure et appliquée’ 


1 We have retained 108 as the atomic weigiie of silver, founding 
our opinion upon a recent observation of Dumas. Stas gave the 
number 107:93. The atomic weights of chlorine, bromine, and 
iodine being dependent upon that of silver, we have also retained 
the round numbers 35:5, 80, 127, as their atomic weiglits. 


94 THE ATOMIC THEORY. 


We would especially draw the attention of the reader 
to the methods employed by Stas in the determination 
of the atomic weights of oxygen, sulphur, chlorine, 
bromine, iodine, nitrogen, potassium, sodium, lithium, 
and silver, giving results the accuracy of which is un- 
surpassed. We must also mention the labours and 
analyses of Marignac, and to the names of the two 
chemists we have just mentioned must be added the great 
name of Berzelius. However, setting aside the question 
of practical chemistry, to which we have just alluded, 
we must confine ourselves to the theoretical discussion 
which has justified the adoption of the new system of 
atomic weights. 

We shall endeavour to show that the atomic weights 
given in the preceding table are in harmony—first, with 
the law of Avogadro and Ampére; secondly, with the 
law of Dulong and Petit; thirdly, with the law of iso- 
morphism. We shall then devote a chapter to the proof 
of the fact that the chemical and physical properties of 
elements are dependent upon the atomic weights. We 
shall prove, lastly, that the notation which is derived 
from the present system of atomic weights ascribes 
to compounds their true molecular magnitudes, and 
allows a correct representation of chemical reactions. 


BOb 


The new system of atomic weights is founded upon 
the law of volumes, and 1s in harmony with the 
hypothesis of Avogadro and Ampére. 

The $ law,’ as it is generally called, of Avogadro and 


LAW OF VOLUMES. 95 


Ampére may be enunciated as follows: Equal volwmes 
of gases or vapours! contain the same number of 
molecules. We have here two things, a group of facts 
and an hypothesis. 

The facts are a result, or rather a development, of the 
laws of Gay-Lassac. 

Gay-Lussac had shown—first, that gases combine in 
simple volumetric relations; secondly, that there is a 
simple relation between the volumes of the combining 
gases and that of the product of the combination. To 
these two laws may be added a third. There is a very 
simple relation between the volumes of all compound 
gases thus formed, and the hypothesis of Avogadro and 
Ampére consists in the assertion that all these com- 
pound gases occupy the same volume, and that the 
matter thus condensed into the same volume exactly 
represents the ultimate particles of the compounds— 
that is to say, the molecules. 

The accompanying table willexplain our meaning :— 


2 vol. hydrogen + 1 vol. oxygen give 2 vol. water. 


2 vol. chlorine + 1 vol. - »» 2 vol. hypochlorous anhydride. 
2vol.nitrogen +1vol. ,,  ,, 2 vol protoxide of nitrogen. 

1 vol. sa + 1 vol. » gives 2 vol. dioxide of nitrogen. 

1 vol. chlorine + 1 vol. hydrogen ,, 2 vol. hydrochloric acid gas. 

1 vol. nitrogen + 3vol. ,, » 2 vol. ammonia. 

1 vol. carb, oxide + 1 vol. chlorine ,, 1 vol. oxychloride of carbon. 
1 vol. ethylene + 1 vol. » 33 1 vol. ethylene chloride. 


We here have clear examples of the two laws of 
Gay-Lussac (see p. 34), as well as of the third law of 
volumes. Between the volumes of the compound 
gases we have the very simple relation 2:1. The 


1 Under the same conditions of temperature and pressure. 


96 THE ATOMIC THEORY. : 


hypothesis of Avogadro consists in the assertion that 
this relation is still more simple, that it is 2 : 2, for the 
smallest quantity or the ultimate particle of oxychloride 
of carbon and of ethylene chloride which can be formed 
does not occupy 1 volume, but 2 volumes. This is au 
hypothesis, if you will, but one the truth of which is 
easily demonstrated, for experiment shows that the 
smallest quantity of carbonic oxide which enters into 
reaction occupies two volumes, which contain a single 
volume of oxygen; it shows, moreover, that the ultimate 
particle or the molecule of oxychloride of carbon cor- 
responds to the ultimate particle or molecule of carbonic 
acid gas, which occupies two volumes. 

These considerations apply to ethylene chloride and 
to other compounds. Consequently it is better to 
express the formation of oxychloride of carbon and of 
ethylene chloride in the following manner :— 


9 vol. carbonic oxide + 2 vol.chlorine = 2.vol. oxychloride of carbon, 
2 vol. ethylene oxide + 2 vol. chlorine = 2 vol. ethylene chloride. 

The two volumes thus formed represent the mole- 
cules of gases or vapours, and we are therefore led to 
give the following form to the statement of the law of 
Avogadro and Ampére. 

The molecules of compounds which are gaseous or 
volatile without decomposition occupy two volumes, if 
an atom of hydrogen occupies one volume. This pro- 
position holds good in the case of by far the greater 
number of volatile compounds, under the condition that 
their true molecular weights are attributed to these 
compounds, 

The proofs are so abundant that it is impossible to 
quote all the examples, and we must confine ourselves 


i lM ~ 7 


LAW OF AVOGADRO. 97 


to giving a list of the groups of compounds which obey 
the law in question. 


Water and its analogues, sulphuretted hy- 
q H,O =2 vol. 
rogen, &c. . . 
Hydrochloric acid and its analogues . HCl = 2 vol. 
Ammonia and its mineral and organic 
analogues; substitution derivatives 


of ammonia; organo-metallic radicals ae = 2 yok 
_of the type RX, 

CLO *~= 3 val. 

ClO; = 2 vol. 

N,O = 2 vol. 

NO = 2 vol. 

Oxides and anhydrides of chlorine, nit- | NO, = 2 vol. 

rogen, sulphur, and carbon : bt BO; = 2 Vol: 

SO, = 2 vol. 

CO = 2 vol. 

CO, = 2 vol. 

LCOS = 2 vol. 

(CH, = 2 vol. 

CH. = .2 vob. 

C.He = 2-vol. 

Allhydrocarbons. . . . ae i" ; sie 

CHa = Zi vol: 

C,,Hig = 2 vol. 

Oi," =" 2 vol: 


SiCl, .= 2 vol. 
PCl, =. 2 vol. 
PCl, = 2Zivol. 
ASCI,..., = 2\vol. 
SbCl, = 2 vol. 
POC], = 2 vol. 


. = 2 vol. 


Chlorides, bromides, and iodides of the 


metalloids and metals - “heen = 2 vol. 
COCL =. 2 vol. 
HeCl. = 2 vol 
SnCl, = 2 vol. 


ALC), = 2.vol | 
2 vol. &c. &e. 


ra 
2 
@ 
a 
l 


98 THE ATOMIC THEORY. 


Mercuric sulphide. ‘ : , . * Het ©* ta vor, 
Alcohols, glycols, phenols. : 5 C,H,O = 2:vol: 
Their anhydrides, ) ethyl oxide. ; (CH). 0 fa ok 
such as Vanek oxide . C370 = 2 vol. 
Aldehydes and aldehyde : » O,H,O)- ='2 vol. 
acetones iste : ; SA SMe 4 8) = 72 vol, 
Organic acids, such as acetic acid « “COCO ee avo, 
Their anhydrides lec anhydride pr i yligus a eee Vor 
succinic anhydride . C,H,O, = 2 vol. 


ethylene acetate (C,H,0,),.C,H, = 2 vol. 
ethyl oxalate . ©,0,(C,H,;), = 2 vol. &c. &c. 


Therlethers ethyl acetate . C,H,0,(C,H,;) = 2 vol. 
7 > 
such as 


This table is undoubtedly very much abridged, but 
it is evident that it embraces a vast number of mineral 
and organic compounds, and it is difficult to imagine 
how, in the presence of such a wealth of facts and proofs, 
accumulated by the labours of the last fifty years, some 
chemists should still refuse or hesitate to believe the 
law of Avogadro and Ampére. It is useless for them 
to bring forward some cases which apparently form 
exceptions, and which we shall presently mention and 
discuss. In fact, we may say that the other physical 
and chemical laws of which we have spoken—the law 
of Dulong and Petit and that of isomorphism—do not 
rest upon such a number of imposing facts, and 
consequently upon such a solid foundation, as the law of 
Avogadro and Ampére. 

When a theoretical idea is true, the exceptions 
which are at first admitted gradually disappear, either 
because the new observations are more accurate than the 
old, or from a more correct interpretation of the facts. 
It also sometimes happens that these exceptions give 
rise to interesting developments of the theory and to a 
more extended generalisation. 


LAW OF AVOGADRO. 99 


That it has been so in the case before us we will now 
proceed to show. ‘ 

I. Thirty years ago ordinary ether was represented 
by the formula C,H,O, which answered to two volumes, 
while the formula of alcohol, C,H,O.HO, answered to 
four volumes of vapour. Here was an exception to the 
law of volumes. Williamson came forward and showed 
that the old formula of ether should be doubled. The 
doubled formula, C,H,,O,, which in the new notation 
becomes C,H,,0, corresponds to that of alcohol, C,H,O, 
both representing two volumes of vapour. It is unne- 
cessary to insist upon the proofs which Williamson has 
given in his masterly memoir, and which are well known 
to all chemists—namely, the existence of mixed ethers, 
and the perfect agreement between the physical proper- 
ties of these ethers and those of ordinary ether, under 
the condition that the latter is regarded as a double _ 
molecule of the form (C,H;,),0. 

II. According to Gerhardt’s notation, which is still 
applied to organic compounds, monatomic hydrates do — 
not contain the elements of water, but merely the residue 
OH. Thus acetic acid is acetyl hydrate, C,H,0.0OH, 
and it is obviously impossible to separate from this 
formula the elements of water, H,O, which could be 
done with the old formula of Berzelius C,H,O, 
=C,H,O0,.H,O, or with the formula in equivalents 
C,H,0,=C,H,0,.H0. Thus it was the opinion of 
Gerhardt that the anhydrides of monobasic acids could 
not exist, and he had the singular fortune to discover 
them himself. But at the same time he showed, in 
striking confirmation of his ideas and formule, that in 


100 THE ATOMIC THEORY. 


order to lose water the molecules of acetic acid must act 
in pairs, one of the molecules furnishing an atom of 
hydrogen, the other the residue OH. The anhydride 
formed, (C,H,0),O, or acetyl oxide, answers to two 
volumes of vapour. 

III. An analogous case occurred with the hydro- 
carbons called alcohol radicals, methyl, ethyl, &c. 
These are imaginary forms, said Laurent and Gerhardt, 
and have no separate existence. Kolbe and Frankland 
isolated them, but showed that their formule must 
be doubled.?- Free ethyl is not composed of two atoms 
of carbon and five of hydrogen, as the group C,H, in 
ethyl hydrate or alcohol, C,H,O.OH, but of C,H,, 
=(C,H,),, and this doubled formula corresponds to two 
volumes of vapour. 

The result of this is that the molecular weights of 
volatile compounds are accurately given by their 
densities. And if we refer these densities. to that of 
hydrogen taken as unity, we have only to multiply the 
numbers obtained by 2 to find the weight of the mole- 
cules compared with that of an atom of hydrogen=1. 
This is a general rule. The density referred to hydro- 
gen is the weight of one volume. 

The molecular weights are the weights of two 
volumes, for molecules occupy two volumes if an 


1 Subjoined is the equation which expresses this dehydration of 
acetic acid— ~- 


— i) + 2s 
of 2 
Two molecules of acetic acid. Acetic anhydride. 


2 «Mémoire sur une nouvelle Classe de Radicaux organiques,’ Ann. 
de Chim. et de Physique, 3° sér., t. xliv. p. 275. 


ia 
Se Oe eee, ee 


LAW OF AVOGADRO. 101 


atom of hydrogen occupies one; we must, therefore, 
multiply densities by 2 in order to obtain molecular 
weights. 

The atomic weights of a certain number of metal- 
loids and metals may be calculated from the molecular 
weights. Thus the atomic weights of phosphorus, 
arsenic, antimony, carbon, silicon, titanium, tin, mer- 
cury, and lead may be calculated from the molecular 
weights of the corresponding chlorides and ethides. 
For example— 

The molecular weight of chloride of silicon (ob- 
tained by doubling its vapour density) is 170, and 
analysis shows that 170 parts of chloride of silicon con- 
tain 142=4 x 35°5 of chlorine and 28 of silicon. The 
vapour density and analysis of chloride of silicon assign, 
therefore, to this body the formula SiCl,, and to silicon 
the atomic weight 28, for we have reasons for the belief 
that the molecule of chloride of silicon only contains a 
single atom of silicon. , 

The vapour density of zinc ethyl doubled = 123, the 
density of hydrogen being = 1. Now, analysis shows 
that these 123 parts of zinc ethyl contain two ethyl 
groups, which weigh 58, and 65 parts of zinc. 65 is 
the atomic weight of zinc, the composition of zinc 
ethyl being expressed by the formula Zn(C,H,),. The 
number 65 (64'9) is, moreover, confirmed by the law of 
specific heats. 

The molecular weight of mercuric chloride, calcu- 
lated from its vapour density, is 271, and analysis shows 
that these 271 parts of mercuric chloride contain 
2x 35°5=71 of chlorine and 200 of mercury. Hence 


102 THE ATOMIC THEORY. 


the simplest composition which can be assigned to mer- 
curic chloride is represented by the formula HgCl,, Hg 
being an atom of mercury. The atomic weight of mer- 
cury is thus fixed at 200, a number which agrees with 
the law of specific heats. 

According to the invaluable experiments of H. 
Sainte-Claire Deville and Troost, the vapour density of 
ferric chloride assigns to this compound the molecular 
weight 325. Now, 325 parts of ferric chloride contain 
213=6 x 35°5 of chlorine and 112 (111°8) of iron. 
Are one or more atoms of iron represented by these 112 
parts? In this case we should no longer prefer the 
simplest hypothesis, as in the preceding cases. The law 
of specific heats attributes to iron the atomic weight 56 
(55:9); we must, therefore, admit that ferric chloride 
contains two atoms of iron, and six of chlorine, and that 
its composition is represented by the formula Fe,Cl,. 

These examples show the use which may be made of 
the law of Avogadro and Ampére in the determination 
of molecular weights and in settling atomic weights. 

We also see the assistance which chemists derive 
from the law of Dulong and Petit, when they have to 
choose between several molecular formule for a given 
compound, and consequently between several atomic 
weights for the same element. 

The considerations mentioned above apply to a 
great number of cases. That this is so will be seen 
from the following table,’ which shows the part played 
by the law of volumes, firstly in the determination of 


1 Abridged from a more complete table which I have given in 
my Lecons de Philosophie chimique, 1864. Hachette. 


LAW OF AVOGADRO. 103 


molecular weights, and subsequently in that of atomic 
weights. The experimental densities given in the third 
column are referred to that of air taken as unity. To 
refer them to the density of hydrogen we have only to 
multiply them by the number 14°44, which expresses 
the relation which the density of air bears to that ot 
hydrogen. The figures in the fourth column express 
the double densities referred to hydrogen, and conse- 
quently the weight of two volumes, 1 standing for 
the weight of one volume of hydrogen. They were 
obtained by multiplying these densities by 28°88. 
They are the same as the molecular weights given in 
the fifth column. Lastly, the sixth column! gives the 
molecular composition: it shows the weights of the 
elements contained in the molecule, and consequently 
the atomic weights, or in some other cases a multiple 
of these weights (see the remarks upon the atomic 
weight of iron, p. 102). The atomic weights thus ob- 
tained from the molecular weights are printed in large 
figures. 


1 T have followed the example of Lothar Meyer 'n adding this 
column, : 


THE ATOMIC THEORY. 


104 


uesoip{y Jol x g=¢E 
woqIvd JO ZT 
eurpor Jo LET 
uasorpdy Jo TL 
outpor Jo LETS 
ussoip{y JjoT x g =e 
wWOqIVd JO ZT 
eulmoig JO O§ 
uosoipsy Jo, x ¢ = € | 
WOqIBd JO ZT 
suTIOTYO JO G.GE 
uasorpsy Jo T | 
ouTLo[YO JO G.GE f 
uas{xo JO 9T x Z = BE) 
wunrtueyes Jo BL J 
uwosfkxo JO 9T X & = QF 
inydyus jo SE 
uadfx0 JO OT X BZ = BE 
imydns jo Z&. 
uasorpsy Jo Z 
inyd{ns jo ZB 
uasorpsy JO % 
uasfxo Jo OL J 


OMoa]OPL Oy} FO worqtsoduroy 


GPL 


86T 


SYUESTO AK 
IT 
Noe[O]L 


——— 


I¥L| §88-F | T0AZ = VHO|* °° eprpor péqo zy 
Sol] StF | TOAG = THe, * ; poe orpormpéy, 
6-€6 | 896-8 | ‘loa g = ig*HO | * = *— oprumorq [AqQey 
Log | ge2't | ‘toag = 1O°HO |* +s opraoqyo pAuOPy 
9¢ LIG-L | “TOA G = TOH |". * pov o1sopqooipé HH 


9IT| €0-4F | ‘ToAg = “099 eprpéque snorusyeg | 
8.6L | $918 | ‘ToAg = *Os epupéyur ormyding 
6-49 | LFBE | ‘ToAs = “OS |*  *_~—- eprxorp epradyng 
FE | SIGLI | ‘OAS = SH |* * oprydins uesoipéy 
SI | ¢629-0 | ‘ToAg = O°H * ae ee eats A) 
BRAY ieee 2 
VRE 
2ESo| ary oF 
~~ t| po«togor SB[NULLO if Sorpog JO soury yy 
4 Sg | sorsueg 
Bins 


105 


PRESENT SYSTEM OF ATOMIC WEIGHTS. 


uesorpéy JOT X $= FU] 
uoqivod Jo SL J 
uasoipdy JOT x ST = ST 
uwoqivo JO ZIT X 9 = ZL p 
oruesie Jo Gs, 
oruesiv JO JZT x § = 18E\ 
otuesie Jo GF, f 
auTIoTYO JO g.cg x § = ¢-90T | 
otuasiv Jo GL f 
uasoipsy JoT xX €=E 
oluesie Jo GL 
eulIopyo Jo g.gg xX € = ¢-90T 
uwasfxo JO 9[ 
snioydsoyd jo Tg 
oulLOTYo Jo g.cg xX E = ¢-90T | 
snxoydsoyd jo TE f 
uesoipsy joT x ¢ = El 
snioydsoyd jo Tg J 


uasoipsy JOT x ¢ = ¢ | 
wogIed JO ZI 
ussoIjlU JO FL 
uesoipsy Jo. x g¢= El 
ussoiyIU JO FL J 
uasAxo JO QT | 
uoZorj1U JO PLS 
uas{xo0 Jo 91 | 
UdSOINIU JO FL x Z = 8aJ 
uasoipsy Jo, xX ¢ = | 
woqivd Jo Z% 
eul0ouy JO T.6 J . 


er 


91 


o9T 


99F 
g- [81 
SL 


g-€91 


g-LET 


ik 


L-4& 


T-91 


691 


6-F9F 
6-I8T 
SLL 


T-€9T 


6-9ET 
GTE 


61-1E 


L0-LT 
86-64 
L-4F 


&-FE 


699-0 


19-7 


ST-9T 


9008-9 
£69-6 


§-¢ 


GPL-Y 
FSI 


80-T 


T6¢-0 
860-1 
LOST 


98T-1 


"TOA G 


"TOA G 


‘JOA Z 
‘JOA G 
‘JOA Z 


“TOA G 


JOA G 
‘JOA Z 


‘JOA Z 


[OA G 


“TOA G 
‘TOA & 


‘JOA G 


ll 


ll 


Svs YSIVv]{ 


* guisivpAqyerqy, 


* @pIpor oluesly 


Oplloyyo oTMesIy 


QuUISIV 


eprmo[yoAxo snioydsoyg 


apoyo snroydsoyg 


sutydsoyg 


* gutuv_syj}oyy 


* eIMOulLy 
OPIXOIp UISOIJIN 


eprxojoid usSo0I4yIN 


epliong [AqIeW 


THE ATOMIC THEORY. 


106 


uasoipfy JOT x 0% = 06 | 
Toag = = 'CH*0)us | Se AES 


moqivd JO ZT X 8 = 96 8-§€6| 9-184} 120-8 | 
uty} JO 8.LTL 
SUHOTYO JOSE FS BYE NR ocag] yy. 6 | JOAZ = Noug |} * °  oepraoryo oruue 
ary Jo S-LITS | 8:93] 2896] 661-6 | T0Az tous PITOTYO oruuEyS 
uasoipséy JOT X 0% = 02 
woqt¥d JO ZI xX 8 = 96 FIT] T-SPI] €0-9 | TAZ = 'CHOVIS |: * — * Oply}o too;]Ig 
WOdTTIS JO 83) 
oullony JO 1-61 X F = F-9L\ yeaa ie: i Ger, 
uoomszo ges] PPOT} 0] 29-8 | Toss AIS | oplony Woorrs 
eullo[yo JO ¢.cg xX F = orl | 2 4 Pal f [ 
moons yo gZs| OL) @-TLT) 686-9 | ‘Toag 10!S opHOTYyo WooI!s 
inydtns Jo Ze x % = F9 | m z ome. 
woqrteo JO BT 92 | FOL | SFOS | ‘OAS gO aprydustp uwoqie9 
aUOTYO Jo g.gg x B= TL) | ‘ 
uaskx0 JO9T ¢; 66 | &86 | 668-E | ‘TOA = 1000 |* = *: SpHopyoAxo woqreD 
woqivo Jo SL J | 
uasfxo0 Jo 9T x 3 = 2S | ; ‘ — : : : 
soqasagesTi(| 1% | LPR | 608-T° |M10s.ge= 00 oplxoIp woqreD 
uasfx0 JO OT : ee ; ; ; 
uoqavo jo BT S| 86 | 648 | 496.0 | Toag = 00 aprxojord woqrep 
ia ae <i ar } | FST| F-991| GIFS | ‘TOAg = 100 | °° —- Splxopyourzzay woqrep 
Seay 
r4 oe 
den 
SHIM) 2B Se} ITY OF 
eTNoITOJWT 99 JO woTyIsodum0g sae lw 2 a=! Lape #eMu40,7 gorIpog JO soumeNy 
os ste 


107 


PRESENT SYSTEM OF ATOMIC WEIGHTS. 


. 


eulIo[yD Jo g°gg X G = GLIT 
umnuspq jou jo 8.Gg 
euIIOTYO Jog.gg x Z = TL 
uashx0 JOOT X G = GE F-SGT 
UINIULOLYS JO $¥.ZG f 
sULIOTYO JO ¢.cg x g= 
qynuIstq Jo O1Z = 
9ULLO[YO JO G.cg xX F = 
wntpeuva Jo Z.TG f 
uadoipAy JOT X GL = QT 


uoqivd Jo ZI X 9 = GL 606 
Auourzue Jo SSL 
aulIopyo Jog.ce x § = ¢.90T | 
AUOUIT}UL JO SSL J 4-866 
uaSoipéy JOT x 6 = 6] 
uoqivd JO GI x § = 9E 9¢ 
uwo10q JO [TL 
suIUOIG JOOS X ¢ = OFZL 192 
uoL0g JO TLS : 
euriony JOLGL * € = $-L¢ 6.99 
uwo10q Jo TT 
auLIOTYO JO ¢.cg X § = 4-901 L ; 
uo1oq JO TL J q-211 
auTIopyo Jos. X F = ZFIL| oer 
mntuv}t} Jo SPS 
gulIo[Yya Jo ¢.¢g xX F = SELL te 
winiu0o1z Jo OB J 
ussoipsy JOT X 9L = 91 
uoqivd JO ZIT xX 9 = ZL 8-S0Z 


ut} JO 8.LT1 


GPT L| z egy | 


“100M 
*10°010 


*1Old 
HOA. 


*CH*oD)ag 
*roqs 
‘CHO a 


"IA 


apltoryoryued umnuepq stoyy 
* eprmoryosxo untu01yg 

; SPHo[you} YNUstTg 

* dplMopyovsjoq tunipeue, 

jive ourqrys[AqoIL], 

*  — sptioTyory} AuouUy 
OpIyAOULlI} WOlOg 
eprumo1q Wo10g 

‘ aprmong wo10g 

‘ @plaqopyo uo1log 

‘ gplorygos wntue4qry, 

: * eplolyo wmniu0011z 


{ © Mad Vice ee 


THE ATOMIC THEORY. 


108 


SA ale ag | = ao8H|* °  eprzorgo ormozeyy | 


uasoipAy JOT x ZL = ZI 
UOGIBD JO ZL X F = BF P| F-99G| B-LLB 96. | Tag = CHOI" ) SPASM eT 
oe 10 9-906 | 
uasoIpAy JOT xX 9 = 
uoqivd Jo ZI xX Z = a4 6-F6 6 Gc-8° | TOA 2 et ec U7 eee [Aqyour OUtZ 
oulz Jo 6 v9 
uesoipsy JOT xX OL = f 
woqivo JO ZI X F = ik 6-661| ZI go-7 | jag = "CHOyaz i". . * - 2 qhqyerouZ 
ue OED I 
sULIOTYO Jo g.cg X g = G-LLT ; eee = . enn dinette 
tanyuiue) Jo BETS | O88] B48 6-31 | [OAS TOAD OploT You} yeyuey, | 
dUTLO[YD JO G.C¢,x E = G.90T | 
waskxo JO 9T P| $913) 82s 88-2 | TAZ = ‘“TOOUN|* *° eplorqyosxo unIqoIN 
wmniqoru 70 #6 | 
osulIO[YyD Jo g.cg xX G = GLLT - : : = : I roTyowjued wntiqor 
“wmnTqoru yo Pe S| THAS| 448 9-6 | ‘TOAZ IOQN OpILOTYOR} IqOIN 


euILOTYyD Jog.cg X 9 = FIZ ; . 
uajs3un} Jo P8T L66 G8E GST | “TOA S 

Pea ds alee SR And . ; = 2 : oplloyyoRyued useyssun 
uajsSun} Jo PST g-19€ 996 L-GT| “JOA G TOM prxopqoey } a 


I 


OM | ° epllo[yoexey usyssuny, 


SF39 
wleeEe 
| aan 
SYSIOM| OB So | ITY OF 
emMosjow oy} Jo worzisodu0g9, Iv Ly | Pettejor e[NUlI0,{ solpog Jo soulvNy 
-Nda[OJL 4g | sorpisuoqg 
alae 
og Act 
Bok 


109 


PRESENT SYSTEM OF ATOMIC WEIGHTS. 


eulIoTyo FO g.cg xX 9 = ae 8.8 6E-IL | * 107a,q *  OplIOTYO OTLI9 iT 


uo jo 6.GG Xx g@ = 8-IIT 


SUIPOT JO C8-9GI ~* oem GOL . we 
ummturuin[e Jo G.LZ x Z = gg L18| 8- 1 TAY, eprIpor UNTUrUINTy 


auTULOI JOOS X 9 = OSF 


. 9 z 
amnyruruinye JoG.L x Z = g¢ seg) L- Ig tv epimoiq WnTUTMN[Ty 


oUTIO[YO JO g.cg x 9 = FI) ae 
cantuyunye Jo 9.45 x o = oof ~TO'lv oplo[yo wWururmany, y 


uasf£xo JO OT X F = Fg ‘ ; 
tmuntuso Jo 9.86L ; : OSO prov OIUISO 


uaSorpAy Jo T X OT a A 
woqivo JO ZI xX F = 893| 8-186 | 16-6 | ‘104g = °CH’D)SH | ° * * Opiyze ONMdTOT 
AIMoIaUL JO wee by 


| uoZorpfy JoT x 9 = 3 
woqivo JO ZI xX Z 0&3| F686 | 628 | Tag = *CHO)MSH|* *  opryjour ormozopy 
Ainoieut jo ooz! 


OUIPOT JO 1ZI x Z = F961 FOF 


. oe z . . : 
Ainoiom Jo OOS S 89F GOL | TOA SG I°H OPIpor OLIMIIO]T 


auIUoId JOOS x Z = O9TL : F it Z ; 6 
fimorom jooozs. 8; =F 9T-GT | T04 G 1g3H Opralorqg oLNoLOW 


110 THE ATOMIC THEORY. 


1 


Apparent Exceptions to the Law of Avogadro 
and Ampere. 


The above method of the determination of molecular 
weights is founded upon the principle that molecules 
occupy two volumes in the state of gas or vapour, an atom 
of hydrogen occupying one volume. Now, the densities of 
a number of vapours are at variance with this proposi- 
tion. Thus, judging from their vapour densities taken 
at a sufficiently high temperature, the molecules of the 
following compounds would occupy four volumes instead 
of two:—Ammonium chloride and similar compounds, 
phosphonium iodide, sulphuric acid, phosphorus penta- 
chloride, iodine trichloride, calomel, amylene hydro- 
chloride and hydrobromide, chloral hydrate, &e. But 
we must endeavour to discover whether the vapours of 
the compounds in question are not decomposed at the 
temperatures to which they are raised in order to take 
their densities, a point to which H. Kopp, Kekulé, and 
Cannizzaro long ago drew attention. If this is the case, 
it is obvious that the densities determined at these 
temperatures do not refer to these compounds them- 
selves, but to the mixture of the products of their de- 
composition. 

_ Thus, for example, we should not be authorised in 
saying that the molecule of ammonium chloride oceupies 
four volumes if it could be shown that at 360°—the tem- 
perature at which the density was taken —this molecule 


« 


DISSOCIATION. Lia 


is entirely decomposed into two new molecules—hydro- 
chlorie acid gas and ammonia gas—which exist side by 
side in a state of mixture, each occupying two volumes. 

It has been proved that this decomposition does 
take place in the case of some of the compounds 
mentioned above, and we propose to give, in some 
detail, the facts and arguments upon which this proof 
rests. 

I. It is unmistakable in the case of amylene hydro- 
bromide, C,H,,.HBr=C,H,,Br. At a temperature which 
is not more than 40° or 50° above its boiling point, the 
vapour of this body presents a density (5:2) which 
agrees with the normal condensation into two volumes, 
and this density is constant between 150° and 180°. 
But from 180° upwards it decreases by degrees till at 
360° it has sunk to one-half. The vapour is, therefore, 
completely dissociated into amylene and hydrobromic 
acid gas, which recombine upon cooling. The same 
phenomena are observed in amylene hydrochloride. 
Nevertheless the vecomposition of the dissociated 
elements is not complete, especially in the case of 
amylene hydrobromide, for, when the flasks are opened 
under mercury, there is always a residue of a certain 
quantity of acid gas, testifying to the dissociation which 
has taken place at a high temperature. There can be 
but one interpretation of this fact. Amylene hydro- 
bromide cannot possess several vapour densities. The 
true vapour density of its molecule is that which 
indicates a condensation into two volumes. - The other 
or halved density indicates a halving of its molecule, and 
is not a true vapour density. It represents a mixture 


112 THE ATOMIC THEORY. 


of decomposition products, and is, as we say, un apparent 
or anomalous vapour density. 

II. The case is the same with phosphorus penta- 
chloride, PC],. It was generally thought a few years ago, 
upon the authority of some very accurate but wrongly 
interpreted experiments of Cahours, that the molecule of 
phosphorus pentachloride answered to a condensation 
of elements into three volumes (H,O=2 vol.) It is 
more correct to say that we have here a phenomenon of 
partial decomposition or dissociation, according to the 
beautiful conception of Sainte-Claire Deville, and that, 
at the temperature at which this vapour is partly dis- 
sociated, of the two molecules which occupy four volumes, 
one is still intact and occupies two volumes, while the 
other is entirely decomposed into phosphorus trichloride, 
PCl,, and into chlorine, Cl,, these products of decom- 
position occupying four volumes; hence the apparent 
condensation of two molecules into six volumes, or of 
one molecule into three volumes. Recent experiments 
have greatly strengthened this interpretation. The 
dissociation of phosphorus pentachloride has, in fact, 
been prevented by diffusing its vapour either into an 
atmosphere of phosphorus trichloride or into an atmo- 
sphere of chlorine. Thus the vapour of the pentachlo- 
ride being formed in a saturated medium of one or 
other of its products of dissociation, the latter is re- 
tarded, the product having become more stable. We 
may conceive, in fact, that the pentachloride being 
dissociated into trichloride and chlorine by heat, the 
trichloride will have less tendency to separate from the 
chlorine in an atmosphere saturated with trichloride, 


DISSOCIATION. rise 


and that, on the other hand, the chlorine will have less 
tendency to separate from the protochloride in an 
atmosphere already saturated with chlorine. Whether, 
therefore, we diffuse the vapour of the pentachloride into 
an atmosphere of the trichloride or into that of chlorine, 
we shall under either condition prevent the decomposi- 
tion of the molecule of the pentachloride into the 
trichloride and chlorine, and shall find that in this case 
the vapour of the pentachloride presents the normal 
density. We are, therefore, authorised in forming the 
conclusion that the molecule of this body offers, under 
these conditions, the normal- condensation into two 
volumes. This also applies to phosphorus pentafluo- 
ride, PFl,=two volumes, which is gaseous at the ordi- 
nary temperature. . 

III. Nothing of this kind is to be observed in 
other compounds, such as phosphorus bromochloride, 
sal ammoniac, and sulphuric acid. At temperatures at 
which their vapours are formed, their molecules are, if 
not entirely, at least to a great extent dissociated ; 
and in confirmation of the statement physical proofs 
may be brought forward from experiments upon dif- 
fusive power, refractive index, coloration, and the ab- 
sorbing power for calorific and luminous rays. Thus 
Wanklyn and Erlenmeyer have shown that when the 
vapour of sulphuric acid, dissociated into anhydrous acid 
and water, is diffused through a tube drawn out to a 
capillary point, the aqueous vapour, being much less 
dense than that of the sulphuric anhydride, escapes 
more easily and in a larger quantity. Sainte-Claire 
Deville has, again, made an interesting experiment with 

6 


114 THE ATOMIC THEORY. 


phosphorus bromochloride, PCl,Br,, which is formed by 
the combination of bromine with phosphorus trichloride, 
thus corresponding with the pentachloride. The colour 
of its vapour was red, thus showing that it contained 
bromine. A similar observation had been made by the 
same author upon the dissociated vapour of the penta- 
chloride, which showed the colour of chlorine. Inthe same 
manner we should be able to determine the absorption 
bands of the vapour of phosphorus pentachloride, which 
should contain those of chlorine. 

IV. An argument of a different kind, but still of a 
physical nature, proves the dissociation of ammonium 
chloride ; Marignac established this fact by showing that 
the quantity of heat necessary for the reduction of sal 
ammoniac into vapour is altogether out of proportion 
with the mean heat required for volatilisation, and strik- 
ingly equal to that which is produced by the combination 
of its elements; hydrochloric acid gas and ammonia. 

V. Ammonium sulphydrate, NH,HS, also does not 
exist undecomposed in a state of vapour. This vapour is 
really a mixture of hydrogen sulphide and ammonia in 
equal volumes, and it appears from Horstmann’s! expe- 
riments, recently confirmed by Salet, that no contraction 
is observable when ammonia is mixed with hydrogen 
sulphide in any proportions between the temperatures 
of 60° and 86°. The assertion of Sainte-Claire Deville 
and Troost that two volumes of ammonia and one 
volume of hydrogen sulphide are condensed to two 
volumes is, therefore, without foundation. 


1 A. Horstmann, Annalen der Chemie wu. Pharm., T. Supplement- 
Band vi. p. 74. , 


DISSOCIATION. 115 


The body generally known as anhydrous carbonate 
of ammonia contains the elements CO,+2NH,. It is 


ammonium carbamate, COC Onit, A. Naumann! has 


shown that its vapour forms six volumes, a mixture of 
two volumes of carbon dioxide and four volumes of 
ammonia. 

VI. Let us pass to another body, calomel. From 
its vapour density we should assign to it the molecular 
weight 235°5 and the formula HgCl, but from other 
chemical considerations the double formula Hg,Cl, 
appears more probable. It corresponds to mercurous 
oxide, Hg,O. Hence we must admit that the vapour of 
calomel is dissociated at the high temperature at which 
its density is taken. And the chemical reactions of 
calomel render this dissociation very probable. It is 
well known how easily it decomposes in presence of the 
alkaline chlorides or iodides into mereuric chloride, 
HgCl,, and mercury, Hg, a decomposition which is 
most accurately represented by the formula Hg,Cl.,. 

The fact of the dissociation of calomel vapour or 
mercurous chloride into mercuric chloride and metallic 
mercury has been demonstrated by Erlenmeyer and A. 
Le Bel. In contact with a platinum tube cooled by a 
current of cold water the vapour deposits metallic mer- 
cury upon the tube. 

VII. As we have remarked above, chloral hydrate, 
C,HCI1,0.H,0, a very definite compound, seems also to 
form an exception to the law of Avogadro and Ampére. 
Its molecule, reduced to vapour, occupies four volumes, 

1 A. Naumann, Annalen der Chemie u. Pharm., t. 160, p. 2. 


116 THE ATOMIC THEORY. 


referred to an atom of hydrogen as occupying one 
volume. But at the temperature at which it is formed 
this vapour is entirely dissociated into a mixture of 
anhydrous chloral, C,HC1,0, and aqueous vapour, H,O, 
the molecules of which each occupy two volumes. In 
the vapour of chloral hydrate the aqueous vapour is 
therefore free, and simply in a state of mixture with 
the vapour of the anhydrous chloral. The author 
proves this by making use of a method mentioned by 
Troost. This chemist heated crystallised potassium ox- 
alate in the vapour of chloral to a temperature of 79°, 
working under such conditions that the tension of dis- 
sociation of the hydrated salt should be equal or a little 
inferior to the tension of aqueous vapour in the vapour 
of chloral hydrate, supposing that the latter were dis- 
sociated. Now, according to the principles developed 
by Sainte-Claire Deville and Debray, the dissociation of 
a body capable of forming a gaseous or volatile product 
ceases for a certain temperature, when this gaseous 
product or vapour has acquired, in the atmosphere in 
‘which it is formed, a certain tension, which is the 
tension of dissociation for that temperature. At 79° 
crystallised oxalate of potassium cannot, therefore, con- 
tinue to produce aqueous vapour, when the atmosphere 
of chloral hydrate contains aqueous vapour under a 
tension which is equal or superior to that of the 
hydrated: salt at 79°. Troost showed that, under these 
conditions, this salt emitted aqueous vapour, and there- 
fore formed the conclusion that the vapour of chloral 
hydrate does not contain aqueous vapour in a state of 
mixture. The experiment was inaccurate and the 


—— 


DISSOCIATION. 117 


conclusion inadmissible. The author has shown that. 
crystallised potassium oxalate behaves in exactly the 
same manner when heated to 79° or 100°, either in an 
atmosphere of chloral hydrate or ina mixture of air and - 
aqueous vapour, and that it does not produce water 
when in the two mixtures the aqueous vapour has the 
same tension, which tension is a little greater than the 
tension of dissociation of the hydrated salt. Indeed, 
anhydrous potassium oxalate can absorb a small quantity 
of aqueous vapour in an atmosphere of chloral hydrate, 
when the tension of the aqueous vapour present in this 
atmosphere is much greater than the tension of dissocia- 
tion of the hydrated salt for the temperature at which 


we are working. 


These experiments leave no doubt as to the condition 
of the aqueous vapour in the vapour of chloral hydrate ; 
it is simply in a state of mixture with the vapour of 
the anhydrous chloral. In fact, if we consider the 
decomposing action which is exercised by heat upon the 
greater number of chemical compounds, we shall not be 
astonished to find that in the compounds under discus- 
sion the point of decomposition is situated below the 
boiling point, or that the latter falls between the more 
or less restricted limits of temperature within which 
the compound suffers dissociation. Hydrochloric acid 
gas has undoubtedly a great affinity for ammonia at 
the ordinary temperature, but at 350° this affinity 
disappears or is very feeble, Marignac having shown 
that the combination between these two bodies cannot 
be effected at this temperature. Again, aniline and 
hydrochloric acid combine at ordinary temperatures, 


118 THE ATOMIC THEORY. 


-accompanied by a large development of heat, but it 
has been shown that there is no development of heat when 
aniline and hydrochloric acid gas are brought in contact 
at a temperature of 230°. 

It appears from the above discussion that the den- 
sities which correspond to four volumes of vapour refer 
to mixtures and not to intact compounds, and that the 
exceptions to the general proposition, that the molecules 
of compound bodies represent a condensation of their 
elements into two volumes of vapour, are more apparent 
than real. 


Vi 


Atomic Constitution of Elementary Bodies in a State 
of Gas or Vapour. 


The law of Avogadro and Ampére not only applies to 
the molecules of compound bodies, but also to the gases 
and vapours of elementary bodies. It is now admitted 
that the latter are formed of more or less complex 
molecules, and that, in the state of gas, these molecules, 
which are contained in equal numbers in equal volumes, 
are situated at immense distances relatively to their 
dimensions, but appreciably constant for different gases 
or vapours. Again, heat, when acting upon these gases 
or vapours, produces in them almost precisely the same 
changes of volume for the same variations of temperature 
and pressure. We cannot here discuss how this idea 
was first introduced into science ; its historical aspect will 
be noticed presently. We will now only remark that 


ATOMIC CONSTITUTION OF ELEMENTS. 119 


the hypothesis of Avogadro and Ampére includes all gases 
or vapours, whether elementary or compound; and if, 
following the proposition given above, which is the 
natural consequence of these ideas, we admit that the 
molecule of compound bodies occupies two volumes, an 
atom of hydrogen occupying one volume, we must also 
admit that the molecules of elementary bodies occupy two 
volumes. Thus a molecule of hydrogen occupying two 
volumes will consist of two atoms, which is also the case 
with oxygen, nitrogen, chlorine, bromine, and iodine. 

All these atoms are diatomic,' and here we again 
meet with the distinction which Gmelin had already 
established between the constitution of the gases or 
vapours of different elementary bodies. This distinction 
has now acquired great importance. 

Ozone is, as we all know, condensed oxygen; this 
has been proved by the experiments of Andrews and 
Tait, and especially by the ingenious and careful re- 
searches of Soret: but ozone is an element, and since 
three volumes of oxygen are condensed into two volumes 
of ozone, which represent a molecule, O,, we may say 
that ozone is triatomic. 

When heated to 500° the vapour of sulphur is still 
more powerfully, though similarly, condensed ; it be- 
comes hexatomic, six atoms of sulphur being condensed 
into two volumes—that is to say, into one molecule—of 
this vapour. Here, therefore, between these two simple 


1 The term diatomic molecules clearly and correctly expresses 
molecules formed of two atoms; but it is clear that this®definition 
of atomicity differs widely frem that which is generally attributed 
to it, ie. the equivalence or valency of atoms. 


120 THE ATOMIC THEORY. 


bodies, oxygen and sulphur, so similar in their chemical 
functions, we have an analogy which deserves our atten- 
tion. Both are capable of condensation, and heat destroys 
this state of condensation. Ozone, when heated, gives 
oxygen, and the molecule of condensed sulphur, S,, is in 
a manner decomposed at a temperature of 1,000°, and 
forms three molecules of vapour of ordinary sulphur, 8,, 
which is diatomic. 

The vapours of phosphorus and arsenic are examples 
of a different state of condensation ; their density, com- 
pared with that of hydrogen, is 62 for phosphorus and 
150 for arsenic. Two volumes of phosphorus vapour 
contain, therefore, 124 parts of phosphorus—that is to 
say, four atoms—and twovolumes of arsenic vapour con- 
tain 300 parts of arsenic, also four atoms. Both vapours 
are tetratomic, or, in other words, the molecules of phos- 
phorus and arsenic are formed of four atoms. Now, it 
has been ascertained that this atomic grouping cannot be 
destroyed by heat, at least not within the limits of tem- 
perature at which experiments have as yet been con- 
ducted ; but this fact does not justify the conclusion that 
it would resist the most powerful action of heat which 
could be produced or conceived, for it is very possible that 
the atoms in question would separate if exposed to the 
temperatures at which iron and platinum melt,and which 
are certainly not so high as those present in the sun. 
What, therefore, it may be remarked in passing, is the 
chemistry possible at the solar temperature? It is 
doubtless very simple and certainly very different from 
terrestrial chemistry. Not to mention the difference of 
elements, can we be sure that known elementary bodies 


ATOMIC CONSTITUTION OF ELEMENTS. 121 


ean enter into combination at the solar temperature, 
or that they produce in other worlds the same com- 
pounds as upon the earth? That is a matter of tem- 
perature. 

To return to the question before us, we now come to 
the simplest of all the molecular groups—that, namely, 
of mercury, cadmium, and probably other bivalent 
metals. The vapour density of mercury is 100, hydrogen 
being taken as unity; but the atomic weight of 
mercury, calculated from the density of volatile mercuric 
compounds (see p. 109), as well as from the law of 
Dulong and Petit, is 200. It follows that a molecule 
of mercury which occupies two volumes contains only one 
atom of mercury. The vapour of mercury is mon- 
atomic. ‘The molecule and the atom of mercury are 
identical, and this consequence of the law of Avogadro 
and Ampére, and which till now has been nothing more 
than a theoretical suggestion, has been recently con- 
firmed by the researches of Kundt and Warburg, the 
principle and results of which we shall now proceed 
briefly to describe. 

It is generally known that the specific heat of gases 
is greater when they are heated under constant pressure, 
and with freedom to expand, than when they are heated 
under constant volume with increase of pressure ; for it 
is evident that, in the first case, the gas must be pro- 
vided not only with the heat necessary to raise its tem- 
perature—that is to say, to augment the mean vis viva 
of its molecules—but also that which is absorbed to per- 
form a certain external work, which would correspond 
to the displacement of an elastic envelope, supposing 


122 ? THE ATOMIC THEORY. 


the gaseous volume to be thus limited. This mechanical 
work is not performed when the expansion is prevented ; 
therefore less heat is absorbed by the gas during the 
elevation of its temperature, It has even been calcu- 
lated, from the mechanical theory of heat, what the rela- 
tion should be between the capacity of gases under con- 
stant pressure and the capacity under constant volume. 
According to Clausius, this theoretical relation is 1°67. 
Now, it appears that for elementary gases, such as 
hydrogen, oxygen, nitrogen, &c., this relation is smaller 
than that indicated by the theory (about 1:4). The 
explanation of this is, that these gases, which are 
diatomic, absorb a certain quantity of heat when they 
are heated under constant volume, not for the perform- 
ance of external work, as there is no expansion, but 
to perform certain work in the molecule itself, which is 
formed of two atoms. 

Now, Kundt and Warburg have shown that this in- 
ternal work is not performed in the case of mercury 
vapour,! and that the relation between the specific heats 
of mercury vapour under constant pressure and under 
constant volume is the same as that indicated by theory. 
It is obvious that in this case there is no internal 


1 Kundt and Warburg have calculated the relation of the two 
specific heats from the velocity of the propagation of sound in mer- 
cury vapour. The calculation was made from the length of a sound- 
wave. In determining the length of a given sound-wave in the air 
and in mercury vapour, they found that the relation of the two specific 


heats of mercury vapour was = = 1:67. (Berichte der Deutschen . 


Chem. Geselisch. zu Berlin, 1875, t viii. p. 945. Poge., Anmn., t. elvii. 
p- 353.) 


ATOMIC CONSTITUTION OF ELEMENTS. 123 


work, because the molecule is only composed of a single 
atom. 

If similar experiments were undertaken for the 
vapours of sulphur, phosphorus, and arsenic, the result 
would doubtless be very different. Here the internal 
work should be considerable, and the relation between 
the specific heats under constant pressure and under 
constant volume would be still smaller than for the 
diatomic gases. 

The distinction which it has been necessary to 
establish between the molecular constitution of the 
different elementary bodies in the gaseous state has now 
been explained, and the significance and value of the 
results given on p. 70 made intelligible. 


Wil: 


The New System of Atomic Weights is in Harmony 
with the Law of Dulong and Petit. 


There is not a single exception to the law of Dulong 
and Petit, as a glance at the following table will show. 
The second column of this table gives the specific heats 
of the elementary solid bodies mentioned in the first. 
The third column gives the atomic weights; the fourth, 
the product of the atomic weights multiplied by the 
specific heats. ‘These products may be termed atomic 
heats, for they represent the quantities of heat absorbed 
by the atoms when their temperature is raised one 
degree. We see that these atomic heats are appreciably 
constant. This constitutes the great physical law dis- 
covered by Dulong and Petit. 


124 THE ATOMIC THEORY. 


Names of Elementary Bodies 


Aluminium 
Antimony . 

Arsenic (cr ystallised) 
Bismuth : 


Boron (crystallised) 


Bromine (solid) 

Cadmium . 

Catton ji diamond at 985° 
\ graphite at 978° 

Cobalt : ; 

Copper 

Gold . 

Indium 

Iodine 

Tridium 

Iron . 

Lead. 

Lithium 

Magnesium 

Manganese 

Mercury (solid) at — 59° 

Molybdenum ; ; 

Nickel 

Osmium 

Palladium. 

Phosphorus (ordinary) at 19° 

Platinum . 

Potassium 

Rhodium . 

Ruthenium 

Selenium . ' ; : 

Silicon at 232°. mae Soe 

Silver ; 

Sodium 

Sulphur 

Tellurium . 

Thallium . 

Tin 

Tungsten . 

Zine . 


Lat 233° 
Lat 600° . 


Specific 
Heats 


0°2143 
0:0523 
0:0830 
0-0305 
0°366 
0°5 (2) 
0:0843 
0:0567 
0-459 
0:457 
0:1067 
0:0952 
0:0324 
0:0570 
0-0541 
0:0326 
0:1138 
0:0314 
0:94.08 
02499 
0:1217 
0:0319 
0:0722 
0:107 
0-0311 
0:0591 
0-189 
0-0324 
0:1655 
0-0580 
0-0611 
0-0762 
0-202 
0:0570 
0:2934 
0-1776 
0-0474 
0:0336 
0:0548 
0-0334 
0-0955 


Products of 
the Atomic 
Weights by 
the Specific 
Heats 


Atomic 
Weights 


ya 


THE OWNIWORNIDAMAEDOAREwWARAwWAT yt apnea 


on 
or 
RRAAADAAAAARAARMABAMAARARARRARMAABPAMAMBRARAIDGRQDT 


DWH AOE DAHNOW 


SPECIFIC HEATS. 125 


The mean of the atomic heats of solid elementary 
bodies is 6°4, and the extreme limits within which these 
atomic heats vary are comprised within the numbers 
5°5 and 6°9. The elements whose atomic heats are a 
little too low are certain metalloids of small atomic 
weight, such as boron, silicon, carbon, phosphorus, 
arsenic, sulphur, and selenium, to which must be added 
aluminium. Those whose atomic heat exceeds the 
average are certain metals, amongst which must: be 
mentioned lithium, sodium, potassium, thallium, cal- 
cium, manganese, melybdenum, &c., to which we must 
add iodine and bromine. But is it not a fact of some 
importance that while the atomic weights vary in the 
proportion of 1 to 30, and the specific heats in the 
proportion of 1 to 7, the products of these two quan- 
tities—that is to say, the atomic heats—only vary in the 
proportion of 1 to 1:2? — 

The variations of atomic heats may be attributed to 
various causes. In the first place, to errors of observa- 
tion connected with the determination of atomic 
weights, and also with that of specific heats. Some of 
these determinations relate to bodies which have not 
yet been obtained in a state of perfect purity. On the 
other hand, as Regnault observes, ' the determination— 
and, we may add, the notion—of specific heats includes 
some uncertainties, ‘for it includes several elements 
which we have not as yet been able to eliminate, espe- 
cially the latent heat of dilatation, and a portion of the 
latent heat of fusion, which is gradually absorbed by 
bodies, as they frequently soften long before the tempera- 

1 Annales de Chimie et de Physique, 3° série, t. xxvi. p. 262, 1849. 


126 THE ATOMIC THEORY. 


ture which is regarded as their melting point is reached. 
Thus the heat applied to a solid body not only serves 
to raise its temperature—that is to say, to augment the 
vibratory energy of its molecules—but a portion, perhaps 
a considerable portion, of this heat is employed in per- 
forming the work of expansion, which work prepares 
the way for a change of state by diminishing the force 
of cohesion, by affecting the disaggregation of mole- 
cules, or by determining modifications of texture. All 
these changes give rise to thermal phenomena, which 
are in some manner superposed, and the sum of which 
constitutes what is called specific heat. It is im- 
possible to distinguish the part played by each of these 
elements in the phenomenon ; but it is surely remark- 
able that, in spite of the compiexity of the phenomena, 
so simple and so great a law should be evolved from them 
when formulated in the terms employed by Dulong and 
Petit. Doubtless it is not rigorously exact, but the 
different elements of which specific heat is composed 
obviously cannot act exactly in the same manner, either 
in different elements or in the same element at different 
temperatures ; and yet these several influences enable us 
to estimate the variations to which specific heat is sub- 
ject, and consequently the atomic heat of certain bodies 
according to the temperature. It is probable that for 
every element there are limits of temperature within 
which the specific heat is almost constant ; experiment 
at least has proved it to be so in the case of certain 
metals, such as iron, copper, zinc, silver, antimony, 
mercury, platinum, lead, and bismuth, and it is to be 
noticed that the atomic heats of these metals approach 


SPECIFIC HEATS. 127 


very closely to the average 6°4. May we not conclude 
from this fact that if the limits of temperature within 
which the specific heat was appreciably constant were 
known for every element, the atomic heats calculated 
from specific heats thus determined would more nearly 
approach the average 6°4? If this were so the law of 
Dulong and Petit, which is only an approximate 
one—as, indeed, are all physical laws, especially the 
law of Marriotte—would acquire a greater degree of 
accuracy. 

However this may be, the variations in specific heat 
are considerable for the three elements which alone 
seemed to form an exception to the law of Dulong and 
Petit—earbon, boron, and silicon. These exceptions have 
just disappeared, for it appears, from the investigations 
of Weber, that the specific heat of carbon, boron, and 
silicon increases with the temperature, and becomes 
constant at high temperatures. This fact has been 
proved in the case of carbon and silicon, and it has be- 
come very probable in that of boron. 

The specific heat of the diamond is very nearly 
0°4589, that of graphite 0°4670; for silicon it is 0°2029 
between 0° and 252°°3; for boron it varies from 0°1915 
to 0°3663 between —79° and 263°6. Weber admits, 
without, however, giving any proof, that it is nearly 0°5 
at higher temperatures. If the specific heats thus 
corrected are multiplied by the atomic weights of 
the three elements in question, we obtain for their 
atomic heats numbers which approximate to the 
average 6°4. 


128 THE ATOMIC THEORY. 


Carbon Silicon | Boron 


| Specificheats: . . .| 0-467 0-203 05 (2 
Atomic weights. : . a ple 28 11 
Al heats : 56 5:7 5‘d 


What explanation can be given of the fact that the 
atomic heat of carbon, boron, and silicon, at low tem- 
peratures, is so far below that of the other solid elements? 
The following consideration will give a clue to the 
interpretation. In order to raise the atomic heat of 
these three elements to the level of that of the other 
solid elements, it would be necessary to multiply their 
specific heat at low temperatures by far higher numbers 
than their true atomic weights; thus, in the case of the 
diamond, it would be necessary to multiply its specific 
heat by 48—that is to say, by the weight of four atoms of 
carbon—before we could obtain a result at all in accord- 
ance with those given by the other solid elements. Can 
it be that this is the atomic weight of diamond, and that 
heat, in acting upon this body, sets in motion aggregates 
of atoms, condensed atoms, instead of acting upon iso- 
lated atoms? The same question arises in the case of 
graphite. To make its atomic heat equal the product 
6:6, it is necessary to multiply its specific heat at low 
temperatures—that is to say, at 200°—by 33. This pro- 
duct 6°6 would represent the atomic heat of graphite, if 
33 expressed its atomic weight. By a coincidence, 
which can scarcely be attributed to chance, this number 
33 has been considered by Brodie as the true atomic 
weight of graphite, which forms, according to this 
chemist, a curious compound with oxygen, in which it 


SPECIFIC HEATS OF GASES. 129 


has the atomic weight 33. This compound of graphite 
has been called graphitic acid.! Similar considerations 
apply also to boron and silicon; they are, however, of 
little importance, since these elements, from the re- 
_ searches of Weber, are now included in the general rule. 

Thus, if the atomic weights given in the table upon 
p- 124 are adopted, there are now no exceptions to the 
law of Dulong and Petit, which is the strongest argu- 
ment we can invoke in favour of the new system of 
atomic weights. 

There is an important fact which should be noticed 
before quitting this subject. The law of specific heats 
only applies to solid bodies. Bromine in a solid state 
conforms to it, but no longer does so when liquid, 
for after liquefaction its specific heat is considerably 
augmented. This is generally true for liquids. It is 
_ well known that the specific heat of water is twice that 
of ice; that of liquid mercury is also higher than that 
of solid mercury, but here the difference is so slight 
(0°0333 and 0:0325) that it does not sensibly affect the 
value of the atomic heat. The state of aggregation 
exercises, therefore, a certain influence upon the absorp- 
tion of heat by the atoms of a body. This influence is 
well marked in the gases; and on this point we will only 
notice one important fact—that the atomic heat of the 
elementary diatomic gases, hydrogen, oxygen, nitrogen, 

1 It is well known that in treating graphite with nitric acid 
and potassium chlorate Brodie obtained a curious acid—graphitic 
acid—in which he admits the existence, not of carbon, but of graphite 
as such. He represents the composition of this acid by the formula 


Gr,H,0,, in which the atomic weight of graphite is the same as its 
thermal equivalent, 33. 


- 130 THE ATOMIC THEORY. 


and chlorine, is sensibly half the atomic heats of the 
solid elements.’ This is evident from the following 
table :— 


| Specific Heats : 
under Constant ae: Products 
Pressure th 


Hydrogen . F : 3°4090 1 3°409 


Oxygen. ‘ ; 0°2175 16 3°286 
Nitrogen . : : 0°2438 14 3°413 


Chlorine . : ; . 0:1210 35°5 4°295 


! 


To obtain the double products, the specific heats 
must be multiplied by doubled atomic weights—that is 
to say, by the molecular weights. 

Molecular Heats.—It has. been admitted as a gene- 
ral rule that equivalent quantities of compound bodies 
possessing a similar atomic composition possess also the 
same specific heat. The products of the specific heats 
of these bodies by their molecular weights are sensibly 
equal, and if this product is called the ‘ molecular heat’ 


1 It is a fact worthy of notice that if the atomic heat of hydro- 
gen and oxygen is calculated from the molecular heat of water (in 
the state of ice) the values obtained will be sensibly equal to those 
calculated from the specific heat of gaseous hydrogen and oxygen. 
In fact, according to the law of Hermann Kopp and Woestyn (p. 132), 
the molecular heat of water, which contains three atoms, should 
equal the sum of the atomic heats of these three atoms, and the quo- 
tient of this molecular heat by 3 should represent the atomic heat 
of hydrogen and oxygen. Now, the specific heat of ice being 0°5, 


: OB x 18_ 9 _ 


If, on the contrary, the atomic heat of the elements of water is cal- 
culated from the specific heat of liquid water, a double value (6) is 
naturally found, which approximates to the mean of the atomic 
heats of the solid elements (6°4), 


MOLECULAR HEATS. 13] 


we may say that such bodies possess the same molecular 
heat, or, in other words, that their molecules absorb the 
same quantity of heat when their temperature is raised 
one degree.' 

As Hermann Kopp has remarked, this law has been 
verified in a great number of cases; thus the nitrates 
and the chlorates NO,R’, ClO,R’, the metaphosphates 
and the metarsenates PO,R’ and AsO,R’, which present 
a similar composition, possess sensibly the same mole- 
cular heat. This is also true for the perchlorates and 
permanganates ClO,R’, MnO,R’, the sulphates and the 
chromates SO,M” and CrO,M”, the carbonates CO,M”, 
and the metasilicates SiO,M”’. The law in question 
appears to be a result of the law of Dulong and Petit, 
according to which the atoms of elementary solid bodies 
require the same quantity of heat to produce a given 
increase in their vibratory energy. Hence similar com- 
pounds containing the same number of atoms must be 
furnished with the same quantities of heat to produce a 
given increase in the vibratory energy of all these atoms. 
Hence, again, the molecular heat must increase with the 
number of atoms of which the molecule is composed. 
This agrees with observation. Upon comparing the 
molecular heats of a great number of compounds, we 


1 M. Regnault has expressed this law in the following manner :— 
The specific heats of compound bodies, possessing similar chemical] 
formulz, are inversely proportionate to their ‘equivalents.’ “(Ann. 
de Chim. et de Phys., 3° série, t. xxvi. p. 264.) 

2 The molecular heat of the nitrates is a little lower than that 
of the chlorates, the metaphosphates, and the metarsenates, a cir- 
cumstance which tends to prove that the atomic heat of nitrogen is 
sensibly below the average.. 


132 THE ATOMIC THEORY. 


find that they consist of the sum of the atemic heats of 
the elements. In fact, the products of the specific 
heats of the molecular weights will at once be seen to 
be equal to as many times 6:4 (mean of atomic heats) 
as the compound body contains elementary atoms. 
Expressing the product in question as c Mm, then 


CM =n x 64.) 


This relation is true for a great number of bodies, 
principally for the chlorides, bromides, iodides, even for 
the complex double chlorides, containing seven or even 
nine elementary atoms,” such as ZnK,Cl, and PtK,Cl,. 
In some cases it may be used as an indirect verification 
of atomic weights. Thus Regnault observed some time 
ago that the molecular heat of tbe chlorides of tin, 
titanium, and silicon is sensibly the same, on condition 


1 Hermann Kopp, Comptes rendus, t. lvi.p. 1254. Woestyn has 
stated the law of the specific heats of compound bodies in a general 
manner by saying that the atom of the element preserves its own 
specific heat in the compound into which it enters. If, therefore, ° 
we represent the atomic weights of several simple bodies by p, p’, p”, 
and their specific heats by ¢, c’, ec”, the products, pe, p’e’, p"e", will 
express their atomic heats. Let c represent the specific heat of a 
compound formed of a certain number », n,n” of atoms p, p’, p”, 
the molecular heat cM of this composition will be 


CM=npet n' po! co! + nl" pl" enon 
and as by the law of Dulong and Petit pe = p’c’ = p” e”, then 
CM=(n+n +n" +...) pe, 


pe being equal to 6-4. This statement expresses the same proposi- 
tion as that given in the text after H. Kopp. 

2 There are, however, some exceptions relative to certain metallic 
sulphides and oxides. (H. Kopp.) 


MOLECULAR HEATS. 133 


that they are represented by similar formule RCl,, in 
which m=5; thus :— 


—— ee 


Products. 


Specific Molecular 
ee Heats Weights (M ‘Hats 
Chloride of tin. ; 4 01413 260 36°7 
ie titanium. : 0°1813 190 34:8 
Pa silicon : : 0:1907 170 32°4 


We see that the products (c M) are sensibly equal, 
which is an argument in favour of the molecular weight 
170 for chloride of silicon, SiCl,, and consequently of 
the atomic weight 28 for silicon. 

The law of H. Kopp and Woestyn, which has been 
illustrated above, may be said to furnish in a great 
number of cases the means of calculating the atomic 
heat of elements from the molecular heat of their com- 
pounds. Jor example, according to Regnault, the 
specific heats of chloride, bromide, and iodide of lead 
are as follows: 0°0664, 0°0533, 0°0427. If these 
numbers are multiplied by the molecular weights 
277°4, 366°2, 460°1 of the three compounds in which 
we suppose 7 = 3, we obtain the products— 

PbCl, 184 © 
PbBr, 19:5 
PbI, 19°6. 

Subtracting from these three products the atomic 
heat of lead, 6°5, we obtain 11°9, 13, 13:1, which repre- 
sent the calorific capacities of the two atoms of chlorine, 
bromine, and iodine contained in the chloride, iodide, 
and bromide of lead. The half of these numbers, 5:9, 
6:5, 6°55, represents, therefore, the atomic heat of 


134 THE ATOMIC THEORY. 


chlorine, iodine, and bromine, and we see at once how 
closely the atomic heat of the chlorine contained in a 
solid chloride approximates to the average 6°4; and this 
seems to be a legitimate conclusion, since the composi- 
tion of chloride of lead is unquestionably the same as 
the bromide and iodide. Finally, the agreement which 
we have proved to exist between the calculated atomic 
heats and those deduced from direct observation for 
Br aud I would not hold if we were to adopt another 
hypothesis for the constitution of the compounds in 
question—for example, if weassumed n=2. This point 
will be developed presently. 

We must add that Regnault was aie to deduce the 
atomic heat of the alkaline metals, potassium, sodium, 
and lithium, from the calorific capacity of their com- 
pounds, and that the values thus calculated are found 
to agree with the results of experiments afterwards made 
with the isolated metals. 

It should also be remarked that the thermal equiva- 
lents of the metals as deduced by Regnault from their 
specific heats are the same as our atomic weights, and 
that this distinguished physicist has long recognised 
the great importance of the law of Dulong and Petit in 
the determination of atomic weights.! 

We will take one more example in illustration of 
the subject now before us. 

Is the atomic weight of mercury 100 or 200? In 
the first case, if we represent 100 parts of mercury by 
Hq, then the mercurous and mercuric chlorides, bromides, 


1 See the note on p. 140. 


MOLECULAR HEATS. 135 


and iodides must be represented by the following 
formule :— 


Mercurous Mercuric 
Compounds. Compounds. 
Hy,Cl ) Hg 
Hg,Br HgBr 
Hg,1 Hl. 


If, however, we represent 200 parts of mercury by the 
symbol Hg, they become— 


Mercurous Mercuric 

Compounds, Compounds. 
Hg,Cl, HgCl, 
Hg,B, HgBr, 
Hg.1, Hel, 


Judging from the specific heats of these compounds, 
the latter system of formulz is to be preferred. In 
fact, if we take 200 as the atomic weight of mercury, 
then in mercurous compounds »=4, and in mercuric 
compounds n= 3, and the molecular heats which may be 
calculated from the formula c M=n x 6°4 are sensibly 
equal to those which are directly deduced from the 
specific heats obtained by experiment. 


Products of 
Specific Heats | Calculated 
Molecular seas 
Formule Specific Heats Weights, ai atime Molemalae 
Hg = 200 eights. Heats, 
Observed Mole- nx 6°4 
cular Heats 


—— | _ 


HeCl, 0:0689 271 18°67 19:2 
Hel, 0:04.20 454 19-06 19:2 
Hg,Cl, 0:05205 471 O4-51 25-6 
| Hees 0:0385 654 25°18 25°6 


The resemblance is here very striking between the 
calculated values and those obtained by experiment. 
This would no longer be the case if the first system of 


136 THE ATOMIC THEORY. 


formule were adopted, and x=3 represented the mer- 
curous and n=2 the mercuric compounds. 


= 
Products of 
Bitivalonte! |!" Hest oy the «| Elenta ot 
: . quivalents. eats he eats of the 
Formule Specific Heats Hg = 100 awtaionté. Equivalents. 
Observed Mole- nm x 3°2 
| cular Heats 
HgCl . 0:0689 135°5 9°33 
HgI 00420 227 9°53 
| Hy,Cl 0-05205 235'5 12:25 
HHg,1 0:0385 327 12°59 


We may close this subject with the following remark: 
—If the considerations drawn from the specific heats of 
the chlorides and bromides of mercury are such as to 


influence our choice between the numbers 100 and 200° 


for the atomic weight of mercury, it is clear, on the 
other hand, that they throw no light upon the question 
as to whether or not the formulz of mercurous com- 
pounds should be doubled. Camnnizzaro prefers the 
simple formula HgCl to Hg,Cl,, because it agrees with 
the vapour density of calomel. We refuse to accept the 
latter formula, because the vapour of calomel presents an 
anomalous density (p. 115), and not from the considera- 
tions drawn from the specific heats; for, in doubling 
the formula, we multiply by 2 each member of the 
equation CM=7”x6°4, which, therefore, leaves the 
question proposed undecided. This is true in many 
other cases. : 

‘Molecular Volatilities.—Bunsen has discovered a 
curious relation between the molecular weights and the 
volatility of certain salts, especially the haloid salts. 
‘If equal weights (one centigramme, for example), of alka- 


a 


MOLECULAR VOLATILITIES, Loe 


line chlorides, bromides, or iodides are exposed in the 
hottest part of the same Bunsen burner, we shall find that 
under these conditions, when the amount of heat fur- 
nished in a given time is sensibly the same, these several 
salts take a very different length of time to volatilise, 
which is almost inversely proportional to their mole- 
cular weights, so that by multiplying the times of 
volatilisation by the molecular weights we obtain 
products which, though not identical, sensibly approxi- 
mate to a mean value (4977). The following table 
gives, according to Bunsen, the times of volatilisation 
in seconds for one centigramme of various haloid salts. 
We have added the corresponding molecular weights, 
and the products of these weights by the times of 
volatilisation :— 


Times of Molecular 


| Salts Volatilisation| Weights Preanors 
Czxsium chloride . A , S13 166 5258 
Potassiumiodide . : é 29°8 165 4934 
Sodium chloride . : ; 84:25 58°5 4929 
Lithium chloride . : F 114°0 42°5 4845 
Potassium chloride . j é 65°4 74:6 4879 
Sodium bromide . : ; 48°8 102°7 5012 
Potassium bromide . ; 41°6 118°8 4942 
| Rubidium chloride : : 38°6 L207 4659 
| Sodiumiodide . . .| 35:7 149°5 5337 
Mean . ATT 


We see that the products vary between the numbers 
4659 and 5337—that is to say, in the proportion of 1 to 
1-4—while the times of volatilisation vary from 29°8 to 
114, in the proportion of 1 to 3°8, This result, though 
only approximate, is significant, and we must remember 
that the difficulties attending the experiment scarcely 

T 


138 THE ATOMIC THEORY. 


admit of greater accuracy. It shows that in an equally 
hot flame, and in a given time, the same number of mole- 
cules of the haloid salts are volatilised, or, in other 
words, that the molecular volatilities are the same. 


Miiis 


The New System of Atonic Weights is in Harmony 
with the Law of Isomorphism. 


The demonstration of this point will be neither long 
nor difficult. We have already observed (p. 60) the 
assistance which Berzelius derived from isomorphism in 
the determination of certain atomic weights, such as 
those of aluminium and iron. The principle which 
guided him in these considerations was correct. The 
atomic weights of simple bodies must be so determined 
that analogous and isomorphous compounds shall receive 
similar formule. This principle is respected by the 
new system of atomic weights. 

We need not here enumerate all the cases of iso- 
morphism presented by the combinations of analogous 
elements, and will therefore only mention the follow- 
ing :—the oxides, 

As,Os, Sb, 0, ; 

the sodium phosphates and arsenates, 
PO,.Na,H + 12H,0, 

AsO,.Na,H + 12H,0; 
the corresponding ammonium salts, 


FP ORCN Hs Ets 
As0,.(NH,),Hs 


LAW OF ISOMORPHISM. 139 


the sulpharsenite and sulphantimonite of silver, 


AsS, A245 
SbS, Ag, ; 
and the isomorphorous phosphates, arsenates, and vana 
dates of the apatite group— 
(PO,),Ca,Cl apatite, 
(PO,),Pb,Cl pyromorphite, 


(AsO,),Pb,Cl mimetesite, 
(V0,),Pb,Cl vanadinite. 


We shall return to the latter compounds. Here we 
need only remark that since the researches of Roscoe 
the atomic weight of vanadium has been altered, so that 
vanadinite, which is isomorphous with apatite, is repre- 
sented by a similar formula; secondly, that calcium and 
lead, bivalent metals in the new system of atomic 
weights, cannot be replaced in the isomorphous com- 
pounds under discussion by univalent metals, such as 
potassium or sodium. 

The latter point is important and requires explana- 
tion. The law of isomorphism teaches us that the 
alkaline metals, amongst which we have included silver, 
because it also is univalent, form a separate group, dis- 
tinctly separated from the several groups of bivalent 
metals, such as magnesium, calcium, barium, stron- 
tium, lead, &c. Thus the sulphate and selenate of 
silver, 

S0,Ag., 

SeO,Ag,, 
are isomorphous with the anhydrous sulphate and 
selenate of sodium, 


140 THE ATOMIC THEORY. 


S0,Na., 
Se0,Na,. 

On the other hand, the alkaline sulphates, selenates, 
permanganates, and perchlorates are isomorphous with 
each other, but not with the corresponding salts of the 
magnesium series. Thus we have— 


SO,KH isomorphous with SeO,KH (Mitscherlich), 


: : CrO,Na, + 10H,0, 
$0,Na, + 10H,O isomorphous with er se é 10H,0. 


The same remark applies to potassium perchlorate 
and permanganate, which, by the new system of atomic 
weights, are represented by similar formulee— 


ClO,K potassium perchlorate, 
Mn0O,K potassium permanganate. 


In the equivalent notation they were written— 


Cl0,.KO _ perchlorate of potash, 
Mn,0,.KO permanganate of potash. 


The isomorphism of the chromates and manganates 
with the sulphates and selenates also deserves notice ; 
it led Berzelius to halve the atomic weights which 
he had formerly attributed to chromium and manga- 
nese. 

The isomorphism of the alkaline chlorides, bromides, 
iodides, and chloroplatinates is so well known that there 
is no occasion to lay stress upon it here. It is admitted 
that silver sulphide and cuprous sulphide! are isomor- 

1 The isomorphism of cuprous sulphide and silver sulphide 
suggested the following reflections to V. Regnault, which are given 
at p. 346 of vol. ii. of his Cows élémentaire de Chimie, 2nd edition :— 


‘Native sulphide of silver is isomorphous with the native sub-sul- 
phide of copper, Cu,S ; these two sulphides seem to have the power 


LAW OF ISOMORPHISM. 141 


phous, at least in their compounds. These sulphides 
are represented by the following analogous atomic 


formulz :— 
Ag,S silver sulphide, 
Cu,S cuprous sulphide ; 


whilst in the equivalent notation they received the 
dissimilar formulee— 


of replacing each other in all proportions, as, for example, in the 
varieties of fahlerze We have maintained that this isomorphism 
exists only between bodies possessing the same chemical formule, 
and we have frequently referred to this law in fixing the equivalents 
of elementary bodies. But sulphide of silver would form an ex- 
ception to the law if we give it the formula AgS—that is to say, if 
we adopt the number 1350 as the equivalent of silver. This con- 
sideration has led several chemists to give to sulphide of silver the 
formula Ag.S, and Ag,O to protoxide of silver, and to take the 
number 675 as the equivalent of silver. This view has been con- 
firmed by several other circumstances which demand our attention 
for afew moments. Physicists have shown by a great number of 
experiments that there exists a very simple relation between the 
specific heats of bodies and their chemical equivalents. This law 
states that the specific heats of elementary bodies, within narrow limits, 
vary inversely as their equivalents. Now, silver will only satisfy this 
law when the number 675 is received as its equivalent. Moreover, 
a law similar to that which we have just indicated for elements has 
been recognised for the specific heats of compounds. This law may 
be thus stated: The specific heats of compounds possessing the same 
JSormula, within narrow limits, vary inversely as the numbers which 
represent the chemical equivalents of these compounds. Now, the sul- 
phides of silver and copper satisfy this law if we represent sulphide 
of silver by the formula Ag,S. 

‘ But if we write the formula of sulphide of silver Ag,S, and con- 
sequently that of protoxide of silver Ag,O, we must write the for- 
mula of soda Na,O, and not NaO, as we have hitherto done, for we 
have seen that the sulphate of silver is isomorphous with the anhy- 
drous sulphate of soda. The salts of potash and of lithia being 
isomorphous with the corresponding salts of soda, when they con- 
tain the same quantities of water of crystallisation, we must give 
to potash the formula K,0 and to Jithia Li,O,’ &e. 


142 THE ATOMIC THEORY. 


AgS sulphide of silver, 
Cu,S sulphide of copper. 

On the other hand, the metals, whose atomic 
weights have been doubled by Cannizzaro, present 
numerous cases of isomorphism, and form a group per- 
fectly distinct from the preceding. The isomorphism 
of the nitrates of barium, strontium, and lead, which 
crystallise in octohedra, is well known, as also of the 
carbonates isomorphous with Iceland spar, which crystal- 
lise in rhombohedra. 

CO,Ca” calcium carbonate (Iceland spar), 
CO,Mg” magnesium carbonate (magnesite), 
CO,Mn” manganous carbonate (diallogite), 


CO,Fe” ferrous carbonate (siderite), 
CO,Zn” zinc carbonate (smithsonite). 


The clinorhombic sulphates and selenates of the 
magnesian series, which crystallise, some with seven, 
others with six, molecules of water, form two isomor- 
phous groups— 7 
SO,Fe’ + 7H,0 ferrous sulphate, 
SO,Co” + 7H,0 cobalt sulphate, 


SO,Mn” + 7H,0 manganouss ulphate, 
SeO,Fe” + 7H,0 ferrous selenate. 


First clinorhombic group. 


SO,Mg” + 6H,0O magnesium sulphate, 
: ; SO,Ni” + 6H,0O nickel sulphate, 
Second clinorhombic S0,Co” + 6H,O cobalt sulphate, 
BIOeP oe Air i 1 SON CH. O zine sulphate, 
SeO,Ni’ + 6H,0 nickel selenate. 


It is a curious fact that, under certain circumstances, 
these same sulphates and selenates can crystallise in 
orthorhombic prisms, and, according tu Mitscherlich, the 
‘sulphates and selenates of nickel and the selenate of 


7 —— 


LAW OF ISOMORPHISM. 143 


zinc (with seven molecules of water) in quadratic 
prisms: they are therefore isodimorphous. 

We must further notice the isomorphous sulphates 
and selenates, which crystallise in the anorthic system 
with five molecules of water— 

SO,Cu” + 5H,O copper sulphate, 
SeO0,Cu” + 5H,O copper selenate, 


SO,Mn” + 5H,O manganous sulphate, 
SeO,Fe” + 5H,0O ferrous selenate. 


Other isomorphous sulphates and selenates crystal- 
lise with four molecules of water in clinorhombic 
prisms— 

SO,Mn” + 4H,O manganous sulphate, 


SeO,Mn” + 4H,0 manganous selenate, 
SO,Fe”’ + 4H,0 ferrous sulphate. 


And, lastly, we must not forget, in the order of com- 
pounds now before us, the numerous double isomorphous 
sulphates of the magnesian series, SO,M”.SO,R, + 6H,O, 
which erystallise in clinorhombic prisms, and in which 
M” may be represented by magnesium, zinc, nickel, 
cobalt, iron, cadmium, or copper, and R’ by sodium, 
potassium, or ammonium, but not by a metal of the 
other group. 

There is no known sesquioxide of the alkaline metals, 
but there are some which are very important and 
characteristic in the groups of metals of which the 
atomic weights have been doubled by Cannizzaro. For 
instance, the sesquioxides of aluminium, iron, manga- 
nese, and chromium present important cases of iso- 
morphism. The following oxides crystallise in the 
rhombohedral system :— 


144 THE ATOMIC THEORY. 


Al,O; corundum, 

Fe,0, specular iron, 

(FeTi)O, titaniferous iron (ilmenite), 
Cr,0, chromium oxide. 


Analogous formulz have been given to all these 
bodies, as well as to their isomorphous compounds, 
amongst which we may distinguish the spinels and 
the alums, which crystallise according to the regular 
system. 

Mg0.Al1,0, spinel, 
FeO.Fe,0, magnetic oxide of iron, 
MgO.Fe,0, pleonaste, 


Zn0O.Al,0, gahnite, 
ZnO0.Fe,0, franklinite. 


The spinels form a very natural isomorphous group, 
and it is a well-known fact that their metallic elements 
often replace each other in the same crystal, without 
change of form. Thus, to take a single example, frank- 
linite forms crystals in which Zn is replaced by Fe or 
Mn and Fe, by Mn. 

It is unnecessary here to draw up a complete list 
of the alums (SO,),M,.SO,R, + 24H,O, in which M is 
aluminium, iron, manganese, or chromium, and R po- 
tassium, sodium, or ammonium. 

The examples which we have just quoted show that 
the new system of atomic weights is in harmony with 
the law of isomorphism; isomorphous elements have re- 
ceived atomic weights which allow us to give analogous 
formule to the similar compounds in which these 
elements occur. In our exposition of the origin of 
this discovery we have already indicated the assistance 
which, in certain cases, may be derived from it in 


ISOMORPHISM. 145 


determining atomic weights, when for a given element 
considerations of a chemical order leave us to choose 
between several values. 

There are, however, a few reservations which must be 
made in connection with the inferences to be drawn 
from isomorphism in the determination of atomic 
weights. 

In the first place we must clearly comprehend the 
definition of isomorphism. All bodies presenting 
identical forms, even with a similar composition, are 
not necessarily isomorphous. In order to be so the 
elements said to be isomorphous must be able to replace 
each other in the same crystal, as, for example, is the case 
with red silver, SbS,Ag, (pyrargyrite), and with proustite, 
AsS,Ag,, with the spinels, garnets, alums, &c. The 
following bodies, though they possess identical forms 
and a similar composition, are not isomorphous, as was 
formerly supposed :— 


NO,Na sodium nitrate, 
CO,Ca calcium carbonate (Iceland spar). 


NO,K potassium nitrate (saltpetre), 
CO,Ca calcium carbonate (arragonite). 


This must not be forgotten: considering the im- 
mense number of chemical compounds and the limited 
number of physical forms which they can affect, it 
must often happen that dissimilar compounds may 
appear under the same form, without authorising us to 
consider them as isomorphous. This, for example, is 
the case with the two iodides of mercury Hg,I, and 
HgI,, which both crystallise according to the quad- 


146 THE ATOMIC THEORY. 


ratic system, and the angles of which are sensibly 
the same, as Des Cloizeaux has recently shown. 
Must we conclude that they are really isomorphous ? 
This seems inadmissible. This conclusion would only be 
legitimate if we were to find well-defined crystals in 
which the two iodides were mixed. 

In the second place, it may happen that compounds 
possessing different atomic structures crystallise under 
the same form and are truly isomorphous. No one will 
deny the isomorphism of potassium chloride and am- 
monium chloride, and of ammonia alum and ordinary 
alum, though the ammonium group NH, presents a 
different atomic structure to that of potassium K. 

Marignac, one of the most eminent and most com- 
petent chemists in these matters, regards as isomorphous 
the double fluorides of titanium, the double oxyfluorides 
of niobium, and the double oxyfluorides of tungsten. 
He has observed that the following double fluorides and 
oxyfluorides, which have potassium or copper as base, 
crystallise under sensibly identical forms :—! 


Potassium salts K,TiF,.H,0; K,NbF,0.H,O; K,WF,O,.H.O. 
Copper salts . CuTiF,.4H,0; CuNbF,0.4H,O ; CuWF,0,.4H,0. 


This is also true for the double fluorides of zinc, which 
form the following series :— 


Zine fluosilicate ZnSikF’,.6H.0, 
Zinc fluotitanate Zn Tif ,.6H,0, 
Zinc fluostannate ZnSnF,.6H,0, 
Zine fluoxyniobate ZnNbOF,.6H,O, 


Zinc fluoxymolybdate ZnMoO,F,.6H,0O. 


) Comptes rendus, vol. 1xxxiv. 


ISOMORPHISM. . 147 


The isomorphism of the fluosilicates, fluotitanates, 
and fluostannates is easily explained by the fact that 
these salts have a similar atomic composition ; but before 
we can bring the isomorphism of the preceding fluorides 
with the oxyfluorides into agreement with the law 
of Mitscherlich we must admit, with Marignac, that 
oxygen and fluorine are isomorphous elements, and 
consequently can replace each other in combinations, 
atom for atom, without producing any change of form. 

Thus we see that the law of isomorphism gives rise 
to some difficulties, and must be applied with judgment 
in the determination of atomic weights. It will only 
prove of real utility, from the point of view in question, 
when restricted to certain groups of analogous bodies, 
and when the conclusions to which it may lead are 
formed under rigorous restrictions. For example, we 
have mentioned above the compounds of niobium, and 
will now add further that Marignac has made a very 
judicious use of the law of isomorphism in fixing the 
atomic weight of niobium and tantalum. The double 
fluoride of niobium and potassium is isomorphous with 
the double fluoride of tantalum and potassium ; tantalic 
acid, Ta,O,, and niobic acid, Nb,O,, should therefore 
receive analogous formule, and the atomic weight of 
tantalum be given as 180 if that of niobium is 94. 
Now, the latter atomic weight has been derived from the 
vapour density of niobium chloride, NbCl,, which has 
been determined by H. Deville and Troost. 

It is in such cases as these that the law of isomor- 
phism affords valuable information, when its indications 
can be connected with the positive intelligence drawn 


148 THE ATOMIC THEORY. 


from the law of volumes or the law of specific heats. 
In the determination of molecular and atomic weights 
the latter laws give more efficient aid to chemistry than 
the law of isomorphism, although the enunciation of 
these laws may not be strictly accurate from a physical 
point of view. We have already made this remark in 
connection with the law of specific heats, which is an 
incomplete law (p. 125). The same remark applies, 
though in a less degree, to the law of volumes. In fact, 
the laws of Gay-Lussac and that of Avogadro and 
Ampére are dependent upon the law of Mariotte, and 
are in a manner forced to follow its variations. 


CHAPTER VI. 


THE NEW SYSTEM OF ATOMIC WEIGHTS RESPECTS AND 
RENDERS EVIDENT THE ANALOGIES WHICH EXIST 
BETWEEN BODIES. 


DUMAS—MENDELEJEFF. 


THE new system of atomic weights renders evident 
numerous analogies which have been discovered in 
chemistry, between either the elements themselves or 
between their compounds or reactions, thus dealing 
with the most varied and the most profound questions 
of science. It is a vast subject, which might be de- 
veloped to a great length, but of which we shall here 
only endeavour to give a sketch. 

Chemistry is not merely an immense collection of 
facts, but moré exactly the science which teaches us to 
classify and arrange them, and this classification should 
begin with the elements themselves. - Attempts have, 
we know, for some time been made in this direction. 


150 THE ATOMIC THEORY. 


The first, which was the most satisfactory, is due to 
Dumas. Admitting the distinction between the metals 
and the metalloids, Dumas proposed to divide the latter 
bodies into five families—namely, those of hydrogen, 
chlorine, sulphur, phosphorus, and boron. The prin- 
ciple, moreover, of this classification—that of comparing 
bodies which form similar* compounds—agrees with the 
natural method. We shall not now dwell upon this 
point, which will be developed presently, but merely 
give Dumas’ division of the metalloids into five families.! 


Ist family: hydrogen. 

2nd family: fluorine, chlorine, bromine, iodine. 

3rd family: selenium, sulphur. Appendix: oxygen. 

Ath family: phosphorus, arsenic. Appendix: nitro- 
gen. 

5th family: boron, silicon. Appendix: carbon. 


Time has made little alteration in this attempt at 
classification. The bodies added as appendices have 
become the heads of their respective. families. The 
only change that has been made has been to separate 
boron from carbon and silicon. Attempts have long 
been made to form the metals into similar groups. 
But here the problem becomes much more complicated, 
because; in the case of a great number of metals, the 
analogies are much less strongly marked, and the extreme 

1 Traité de Chimie appliquée aux Arts, t. 1., Introduction, p. 
Ixxvii. We owe the term metalloids to Simon,who proposed it in 
1808 to designate the metals of the alkalis and of the earths re- 
sembling the metals properly so called. In 1811 Berzelius applied 


the term to the non-metallic elements. (H. Kopp, Geschichte der 
Chemie, t. iii. p. 96.) 


‘ 
: 


CLASSIFICATION OF ELEMENTS. 151 


terms of each group disagree to some extent in their 
properties and in the nature of their compounds. Thus 
certain metals constitute a transition between the several 
groups, and these intermediary terms only serve to put 
difficulties in the way of classification. Nevertheless, 
several groups of metals have been established. We 
will mention, in the first place, the alkaline metals, to 
which may be added, as an appendix, silver, thallium, 
and, to a certain point, copper and gold. 
A second group comprises— 


Calcium 
Strontium 
Barium, 


to which may be added on the one hand— 


Lead, 
on the other— 


Macnesium 
Zine 
Cadmium. 

Cobalt, nickel, iron, and manganese are connected 
with the preceding series through zinc. But this group 
throws some difficulties in the way of classification, as 
iron and manganese, which offer some analogies with the 
metals of the magnesium series, are connected, from 
another point of view, with both chromium and 
aluminium. 

Another group of metals is connected with silicon, 
and comprises— 


Titanium 
Zirconium 
Tin, 


152 THE ATOMIC THEORY. 


The following metals are connected with the family 
of nitrogen, phosphorus, and arsenic :— 


Vanadium 
Antimony 
Bismuth 
Niobium 
Tantalum. 


Molybdenum and tungsten present many mutual 
analogies, and resemble chromium and uranium, 

Copper is difficult to classify. In the nature of its 
compounds it is not unlike mercury, but it also pre- 
sents analogies with silver, and from this point of view 
resembles the alkaline metals. 

Finally, the metals which accompany platinum have 
always been grouped into one family, which may be sub- 
divided into three classes—i.e. ruthenium-osmium, 
rhodium-iridium, palladium-platinum. 

The metals which form part of these families or 
classes are characterised by the analogy of the com- 
pounds which they form with the metalloids, particularly 
with oxygen and chlorine; for here, unfortunately, 
hydrogen compounds are wanting. Similar formule 
are accorded to a given group of the compounds in 
question, if appropriate atomic weights are assigned to 
the latter. 

Each group of metals differs from the rest in the 
nature of its compounds. This is an established fact, 
and will be developed presently. But. it was formerly 
unknown, having only recently been discovered, that 
the characteristic properties of the elements, which 


CLASSIFICATION OF DUMAS., 153 


determine the nature of their compounds, are dependent 
upon the atomic weights. 

Chemists such as Gladstone, Cooke, Pettenkofer, 
Odling, Kremers, and Dumas had pointed out certain 
numerical relations existing between the atomic weights 
of bodies belonging to a given group. | 

Thus, to quote a few examples from Dumas, very 
simple relations exist between the ‘ equivalents’ of 
the bodies belonging to the families of oxygen, 
lithium, and magnesium. The numbers expressing 
these ‘equivalents’ form part of arithmetical pro- 
gressions a+ ad. | 


Oxygen. Sulphur Selenium. Tellurium. 
a= 8 8 16 40 64 
d= 8 a at+d a+ 4d a+ 7d 
Lithium. Sodium. Potassium, 
a= 7 7 23 39 
a@d=2-x 8 a ard a+ 2d 
Magnesium. Calcium. Strontium. Barium. 
a= 12 12 20 44 68 
d= 8 a a+d a+ 4d a+ 7d 


We shall notice, as Pettenkofer has already done, 
that in these three families, the differences between the 
equivalents of the analogous elements are represented 
by 8 or by a multiple of 8. 

In the families of fluorine and nitrogen we meet 
with the following relations, which are not quite so 
simple :— 


Fluorine. Chlorine. Bromine, Iodine, 
a= 19 19 35°5 80 127 
ad = 16°5 a a+d a+2d+d' 2a+ 2d + 2d 


ad = 28 


154 THE ATOMIC THEORY. 


Nitrogen. Phosphorus. Arsenic. Antimony. . Bismuth. 
a= 14 14 31 75 119 207 
a@=17 a a+d at+d+d' a+d+2d a+d+4d' 


The comparison of these numerical relations has led 
to an ingenious inference. If the homologous radicals of 
organic chemistry are formed by the addition of nCH, 
to a given compound, why should we not suppose that 
the metals themselves are formed by the addition of a 
given species of matter, differing only in the manner of 
condensation? This was Prout’s hypothesis, which 
reappears under another form, but even when thus 
transformed does not admit of a definite conclusion. 
Nevertheless we may derive from all these facts and 
considerations the following conclusion: the properties 
of bodies are dependent upon the atomic weights, and 
when we observe a great resemblance between a certain 
group of elementary bodies we shall also find a certain 
regularity in the increase of their atomic weights. 


II. 


The work of Mendelejeff has lately thrown a new light 
upon the relations existing between the atomic weights 
of elements and their properties. The latter are a 
function of the atomic weights, which function is 
periodic. Such is the proposition of the Russian 
chemist. It is not limited to such and such a group of 
elements, but embraces all the elementary bodies of 
chemistry. It is not limited to the consideration of cer~ 


MENDELEJEFF’S PERIODIC LAW. 155 


tain analogies, but comprises all physical and chemical 
properties. It is simple in its principle and productive 
in its results. All the elements are arranged according 
to the increasing value of their atomic weights. We 
thus find that from one element to another the figures 
expressing these atomic weights only differ by a few 
units. We also remark that the properties are gradually 
modified as the atomic weights increase; that these 
modifications, moreover, do not advance continuously 
from the first term to the last, but pass through several 
cycles or periods. The differences between the atomic 
weights of contiguous elements are appreciably equal, 
but not absolutely so; and even in some cases we find 
very considerable discrepancies, as if there were a gap 
between contiguous elements. Mendelejeff has pointed 
out several, and it is aremarkable fact that one of these 
gaps has since been filled up. Lecoq de Boisbaudran’s 
gallium had its place assigned in Mendelejeff’s list. Its 
density had been accurately foreseen from the number, 
which was very near the truth, assigned to its atomic 
weight. The synthesis of the Russian chemist is thus a 
powerful one, and must in future be taken into considera- 
tion whenever we undertake a classification of bodies in 
accordance with their properties and reactions, or, in a 
word, regard the facts of chemistry from a lofty and 
comprehensive point of view. 

The following example will explain Mendelejeff’s 
conception :— 

Let us take the fourteen elements whose atoms are the 
lightest after that of hydrogen, and arrange them in two 


156 THE ATOMIC THEORY. 


horizontal lines, following the progression of their atomic 
weights. 
Pees id = PAs Boe 1150 = 125 Nia 145°0 = 165 


Na = 23; Mg = 24; Al = 273; Si = 28; P=81; § 
Cl = 35°5. 


19. 
32¢ 


I 


In these two groups of simple bodies physical proper- 
ties and chemical characters manifest gradual modifica- 
tions proportional to the increase in the atomic weights. 
Thus the densities increase regularly, so that they reach 
a maximum about the middle of the series and after- 
wards diminish. Again, the atomic volumes—that is to 
say, the volumes which would be occupied by quantities 
proportional to the atomic weights, and which are the 
quotient of the atomic weights by the densities— 
naturally follow an inverse proportion to that of the 
densities ; they decrease regularly, reaching a minimum | 
about the middle of the series (see the table further on). 

Thus, taking only the second group (the three last 
terms of the first are gaseous), we have, for the different 
simple bodies of which it is composed, the following 
elements and atomic volumes :— 


Na Mg Al Si 2 s Cl 
Densities O97. 1:75 2°67 2°49 1:84 2°06 1:38 
Atomic vol. 24 14 10 11 16 16 27 


We find, moreover, that the volatility diminishes 
from sodium to silicon, and increases again after silicon. 
The chemical characters of the metals belonging to 
these groups are also subject to regular variations. 
Between each term differences are found in the funda- 
mental chemical properties, which differences are 


MENDELEJEFF'S PERIODIC LAW. 157 


manifested by the nature of the compounds. Among 
these compounds Mendelejeff has considered those 
formed with hydrogen or chlorine, and especially with 
oxygen. In these two groups the three first terms do 
not form combinations with hydrogen; such combina- 
tions do, however, occur in the last four, and here we may 
note the remarkable peculiarity that in these hydrogen 
compounds the number of hydrogen atoms decreases regu- 
larly from four to one (see the following table). 

As we have just remarked, the metals which form 
the first terms of the two preceding groups do not enter 
into combination with hydrogen; they unite, on the 
other hand, with chlcrine, and their capacity of combi- 
nation with this element increases regularly. This 
double variation is shown in the following table :— 


FH 
ClH 


LiCl Gcl, 
NaCl | MgCl, 


BCI, 
AlCl, 


CCl,; CH, 
SiCl, ; SiH, 


NH, 
PH, 


OH, 
SH, 


A similar regularity may be observed in the oxygen 
compounds: the number of oxygen atoms, with which 
the metals forming the two groups can combine, increases 
regularly from the first to the last.! 


——» 


5,0," 


oo 


C10, 


Li,0 | G,0,* BO, 
Na,O | Mg,0,* | Al,0, 


C,0,* 
Si,0,* 


NO, 
P.O, 


Another interesting peculiarity consists in the fact 
that the chemical functions of all these oxygen com- 
pounds are gradually and regularly modified from term 
to term, the first being strong bases, the intermediate 
ones indifferent bodies, and the last powerful acids. 


1 The formule marked with an asterisk have been doubled, so as 
to make the regularity in question more striking. 


158 THE ATOMIC THEORY. 


Now, the chief characteristic of all these variations 
is the fact that they are repeated in both growps, so that 
the first term of one agrees with the first term of the other 
(Li with Na), the second with the second (G with Mg), 
and soon. Supposing all these elements to be arranged 
one after the other, we distinguish, as far as the variations 
of properties are concerned, two periods, the one begin- 
ning with lithium, the other with sodium. This is what 
Mendelejeff calls the periodic law. He extends it to all 
elementary bodies, and expresses it in the following 
terms: the properties of elements (and consequently 
those of the compounds which they may form) stand 
in periodic relation to their atomic weights. 

In the following table all the simple bodies are 
arranged according to the progression of their atomic 
weights, and, in addition to this, are made to form hori- 
zontal and vertical series. 

The horizontal series consist of elements resembling 
each other in their atomic weights, and of which the 
properties are gradually modified, so as to complete the 
period. 

The vertical series consist of elements connected by 
the whole of their properties, and which may be termed 
homologous. The elements given in these vertical series 
form natural families. 


159 


MENDELEJEFF'S PERIODIC LAW. 


6-6 9-8 6-6 oe TL ST L1G — | 698 1-99 |* ‘JOA oruL0yy 
g-IT L-3L 6-1 val 9-8 16-9 ST-7 rig 09-4 CPL 1° solpisuaq 
6-901 é-FOL ¢-€0I = 8-96 ¥6 06 9-68 6-18 6-8 | S}USIOM ‘WOZy 
bd uu ny d ot aN IZ AY Ph aS qu 
| 
ee ee ee | ee) eee | eS eee | eee 
6:96 6-91 6-E1 oe LUT 1-6 GL * "TOA O1UL0 WV 
16-6 9-F L9-9 “= 96-9 SIL 8-8 : sol}Isuagq 
GL-61 82 6-FL GL 6-69 6-49 €-69 | s}ys1om ‘uL0yy 
Ig oS sy j ep az? |) nO 
1-9 6-9 GL 6-9 Leh €-6 sy ro, ¥-S3 FCF |° ‘OA O1MI0TYV 
8-8 ¢-8 8-L 8 8-9 g-9 “ 2 Lg-T 98-0 ‘ solpIsuog 
9-89 9-8¢ 6-99 8-49 ¥-69 61g St * 06-68 FL-68 | S}Staa “W107 
IN 00 oH ul 1 A LAF d L®) M 
L-&6 L-ST g-€T 6-11 L-OT 8-1 LEG | ° ‘TOA oTULOZ Vy 
8é-T 40-6 6G 92-6 6F-3 | FLT 16-0 : ST} ISUa(T 
. 3-9§ 6& Ig 86 6-16 | 43 €Z | S}ystom ‘woz | 
10 S d 1g IV | Iq eN 
-— sme — 9-8 L-¥ ¥-F GIT |° *[OA o1uL0yV 
i. re. a iE 89-6 | 1:36 69-0 : satqisud(] | 
T-6L | 96-91 | ¥0-FI a IT €6 0-2 S}YSIOM “UOT 
i 0 N 6 d 9 YI | Fan 


6 


q 


THE ATOMIC THEORY 


160 


§-6 36 6-6 
GI-16 GI-16 ¥16 

L-961 L-961 9-861 

Id qT 80 


TT ¥-08 — — — 
€-8I a, — — — 
i OFZ 6-83 = — — 
ial UL i j 
es T-81 TT 1-41 Z-OL 
686 €8-II | 98-11 6S-81 ¢-61 
O12 | ¥F-906 | 9-802 002 6-961 
Iq ad LL Sy ny 
9-6 6-91 — — — — 
€1-6L | 48-01 — = — — 
+8 Z8T — 9-OLT a — 
M el, i Iq | } 
— — — ee C.98 — 
— — — — G1.g — 
L¥I are GSI LEI 8-981 | S1-ZST 
Iq i eT 99 eq so 
C.0Z Z-8I1 T-91 ¢-ST 6-21 ¢-0T 
63-9 1-9 63-1 GF-L 69.8 GOT 
183 Gol 8-211 FEL | 9-L1T SOL 


‘ “TOA oTml0yy 
: sorjyisuagy 
S]USIOM "WOLY 


* "JOA OTm0y VW 
; sorqIsuog 
S}YSTOM ‘010; y 


[OA DIULOT WY 
. SOT}ISUO(T 
S}YSIOM “UL0YW 


"[OA OTULOZY 
: Sor}IsUa(y 
S]USIOM "UO; ¥ 


" “JOA OTUIOyW 
SOTISUa(T 
S}YSIOM ‘Ul0yY 


MENDELEJEFF’S PERIODIC LAW. 161 


We have given this table at length, that the reader 
may estimate the true value of the attempt at classifi- 
cation in question, which, for the first time, embraces 
all the elements known to chemistry. This attempt, 
doubtless, still presents many imperfections, greatly due 
to the uncertain state of our present knowledge, especially 
with regard to rare elements. Thus tellurium is not in 
its place, supposing its atomic weight to have been 
accurately determined. If tellurium were the inter- 
mediate element between antimony and iodine it should 
possess an atomic weight of about 125. A question might 
also be raised as to whether copper is correctly placed : it 
is separated from certain elements—mercury, for example, 
which it appears to resemble. Other simple bodies, such 
as cobalt and nickel, the atomic weights of which are very 
similar, if not identical, also give rise to a difficulty. 
According to the principle of the classification, their pro- 
perties should similarly coincide, which is not the case. 
And yet we know that these metals have many points 
in common. This is also the case with chromium, 
manganese, and iron, which are placed side by side in 
the same horizontal series, and between the atomic 
weights of which there is very little difference. On 
the other hand, great differences may be observed be- 
tween the properties of vanadium and bromine, between 
potassium and calcium, between rubidium and ruthe- 
nium, which yet are so closely related by their atomic 
weights. - In the same manner we must confess that 
the variations or gradations of properties are far from 
progressing regularly or uniformly in the different 
groups. In some cases they are too great, as in the 

8 


162 THE ATOMIC THEORY. 


first group, carbon, nitrogen, oxygen, and fluorine; in 
others too slight, as we have just remarked, for the last 
terms of the third group. Though it may be gene- 
rally true that the properties of bodies are subject to 
periodic modifications with the increase of their atomic 
weights, the law of these modifications escapes our 
observation, and seems to be of a complicated. nature ; 
for, on the one hand, the atomic weights of. succes-. 
sive elements vary within considerable limits, without 
displaying any regularity in these variations; on the 
other hand, we must confess that the gradations of 
properties, or, in other words, the greater or less diver- 
gencies between the properties of successive elements, 
do not appear to depend upon the degree of the dif- 
ferences between the atomic weights. These are real 
difficulties. 

In the preceding table we are principally struck, at 
first sight, with the gaps which may be noticed between 
two elements, the atomic weights of which show a greater 
difference than two or three units, thus marking an in- 
terruption in the progression of the atomic weights. 
Between zinc (64°9) and arsenic (74:9) there were two, 
one of which has been recently filled up by the dis- 
covery of gallium. We must, however, remark that 
the considerations by which Lecog de Boisbaudran 
was led in the ‘search’ for gallium (for this great dis- 
covery is not due to chance) have nothing in common 
with the conception of Mendelejeff. Again, though 
gallium has filled up a gap between zinc and arsenic, 
and though other gaps may be subsequently filled, it 
is by no means proved that the atomic weights of the 


MENDELEJEFF'S PERIODIC LAW. 163 


new elements will be those assigned to them by the 
principle of classification which we have been discus- 
sing. 

In fact, the atomic weight of gallium is sensibly 
different to that which was predicted by Mendelejeff. 
It is also possible that the future may be reserv- ' 
ing for us the discovery of a new element, the atomic 
weight of which will closely resemble or coincide with 
that of a known element, as the atomic weight of nickel 
coincides with that of cobalt, and as that of potassium 
closely resembles that of calcium, and such a discovery 
would not fill any foreseen gap. For example, if cobalt 
were unknown, it would not be discovered by Mende- 
lejeff’s principle of classification. This imperfection is 
undoubtedly due to the fact mentioned above, that the 
rate of increase in the atomic weight of elements 
belonging to the same period (horizontal series) is alto- 
gether irregular. 


ELT. 


Among the physical properties dependent upon 
atomic weight we have not yet mentioned density. Other 
physical properties seem in the same manner to be sub- 
ject to periodic variations with the increasing value of 
the atomic weights. We may mention particularly 
malleability, fusibility, volatility, and conductibility for 
heat and electricity. Without entering into the details 
of this subject, we may give an outline of all the 
facts, drawing our information from a graphic con- 
struction for which we are indebted to Lothar Meyer 


164 THE ATOMIC THEORY. | 


who has contributed a detailed and important deve- 
lopment to Mendelejeff’s idea. (See the end of the 
volume. ) 

The elements are arranged upon the axis of the ab- 
scisse, at distances from zero proportional to their atomic 
weights, each element occupying a fixed point upon the 
axis. At this point an ordinate is drawn, which repre- 
sents the atomic volume of the given element. The 
curve which joins the extremities of the ordinates repre- 
sents, therefore, the variations of the atomic volumes. 
From the absence or uncertainty of the data relative to 
certain gaseous or other little studied elements, it has 
been impossible to give the entire curve. In particular 
an important gap is visible between didymium and tan- 
talum, and in other places dotted lines are used, where 
certain unknown atomic volumes are interpolated.! 
This being granted, the graphic construction shows at 
once that the variations of the atomic volumes (and 
consequently of the densities) are periodic. Starting 
from lithium, the curve sinks till it reaches a mini- 
mum which corresponds with boron; it then rises, 
attaining a second maximum with sodium. At this 
point it descends again, then rises to a third maxi- 
mum with potassium, and so on. Now it is proved 
that the position occupied by the elements upon this 
curve is in relation with their physical and chemical 
properties. 

In the first place, as far as the densities are con- 


1 The atomic volumes of elements may be indirectly determined 
by deducing them from the molecular volumes of their liquid or 
solid compounds (see Chapter VII.) 


. 


MENDELEJEFF’S PERIODIC LAW. 165 


cerned, it is evident, from the very principle upon which 
the curve is constructed, that the light metals (posses- 
sing considerable atomic volumes) should occupy the 
maxima, and the heavy metals (possessing low atomic 
volumes) the minima; but the fact which particularly 
demands our attention is that, with atomic volumes 
sensibly identical, two metals may possess very different 
properties, as they are situated upon the ascending or 
descending portion of the curve. | 

The ductility, fusibility, and volatility of elements 
are related to their atomic weights, and are subject 
to periodic variations with the increase of their atomic 
weight. The light metals, which occupy the summits, 
or the immediately succeeding descending portion of the 
curve, are ductile. The heavy metals, occupying the 
minima, or the ascending portion near the minima, of 
the curve, are partially ductile in the fourth, fifth, and 
sixth groups.’ Take, forexample, the fourth, which com- 
prises the elements placed, from the progression of their 
atomic weights, between potassium and rubidium. The 
light metals, potassium and rubidium, which stand at 
the top of the curve, are ductile. A decrease should be 
observed in the ductility of the elements placed upon 
the descending branch, till at the bottom we meet with 
brittle metals, such as vanadium, chromium, and man- 
ganese. From iron, which follows, the ductility in- 
creases with the elements which occupy the minima, or 
the immediately sueceeding ascending branch. Ductile 
copper is the last of this ascending series. With 


' The three first groups only contain heavy metals. 


166 THE ATOMIC THEORY. 


gallium the ductility again decreases; arsenic 1s 
brittle. 

Thus we see that in the fourth group of elements, 
while the density increases and diminishes regularly 
with the increase of the atomic weight, from potassium 
to rubidium, the ductility diminishes and increases 
twice. Thus the variations of ductility extend to two 
groups instead of one, as is the case with density. It 
also appears that elements which have evidently the 
same atomic volume, such as chromium and copper, 
vanadium and zine, differ to a striking extent in duc- 
tility. Vanadium and chromium, situated upon the de- 
scending branch of the curve, are brittle ; copper and zinc 
are ductile, though in a different degree. And since we 
have drawn attention to the elements ranged in group 
IV between potassium and rubidium, we may remark 
that there wds a considerable gap between zine (Zn 
= 64:9) and arsenic (As=74°9). It was here that Men- 
delejeff placed his ‘ekaluminium,’ which is the gallium 
of Lecoq de Broisbaudran. 

From the place of this element, between zine and 
arsenic, though nearer to zinc, Mendelejeff was able to 
predict that its density would be about 5°9. Now 
Lecoq de Boisbaudran has found it to be 5:96. From 
the place occupied by gallium in the third vertical 
series on p. 159 the eminent Russian chemist was able to 
discover a connection with aluminium, which is found 
to be correct ; thus gallium oxide resembles aluminium 
oxide. 

We should be exceeding the limits which we have 
imposed upon ourselves in this treatise, if we gave fresh 


- 


MENDELEJEFF’S PERIODIC LAW. 167 


‘developments and examples of the relations which exist 


between the atomic weights and other physical proper- 
ties. It must suffice to say that fusibility and ductility 
are, with the progression of atomic weights, subject to 
variations similar to those manifested by ductility and 
density. Crystalline form and expansibility by heat 
appear also to be dependent upon the atomic weights. 
Fizeau’s careful researches upon the coefficients of ex- 
pansion of a certain number of simple bodies '! is well 
known. The results obtained by the eminent physicist 
show that this coefficient increases and diminishes 
regularly, as the atomic weight rises. Here, again, we 
observe the periodicity in the variations of properties, 
which is the striking characteristic of Mendelejeft’s 
law.? 

The relations which exist between atomic weights 
and specific heats were discovered by Dulong and Petit. 
We gave them on p. 124, observing that the atomic 
heats are not precisely identical, but that the law of 
Dulong and Petit is subject to irregularities, and that 
the latter are in a certain measure due to the degree 
of impurity, to the want of homogeneity in the solid 


1 Comptes rendus, vol. Ixviii. p. 1125. 

2 L. Meyer has illustrated the influence of the atomic weights 
upon the expansion by heat in a table similar to that upon p. 159, 
but in which the vertical series are so disposed that the three last 
terms of the third group, Fe, Co, Ni, become the first of the fourth. 
For these developments we refer our readers to the excellent work 
of Lothar Meyer. The same chemist has pointed out and discussed 
the relations which exist between the atomic weights and the coeffi- 
cient of refrangibility, the conductibility for heat and electricity 
( Die modernen Theorien der Chemie, Breslau, 1877.) 


168 THE ATOMIC THEORY. 


bodies, and to the variations to which the specific heats- 


are subject in a given body, according to the tempera- 
ture. But Lothar Meyer has remarked that the ele- 
ments which only approximately obey the law of 
Dulong are generally those of which the atomic weights 
as well as the atomic volumes are low.. This is the case 
with carbon, boron, and silicon. 

Amongst the elements with low atomic weights 
which nevertheless obey the law of Dulong, must be 
placed, lithium, sodium, magnesium, and potassium, 
which, on the other hand, possess a high atomic volume— 
that is to say, their density is low. We are, therefore, 
led to conclude that the irregularities to which the law 
of Dulong and Petit is subject are not only due to 
the causes which we have enumerated above, but are 
also related to the different volumes occupied by the 
ultimate particles of bodies—that is to say, to the 
atomic volumes. Atoms which occupy the smallest 
volumes have a lower specific heat—in other words, re- 
quire a little less heat in undergoing. the same varia- 
tions of temperature than the more ‘ voluminous’ atoms 
of other elements. 

Be this as it may, Dulong and Petit were the first 
to show: that the specific heats of simple bodies are 


dependent upon the atomic weights, for they decrease . 


regularly as the latter increase. And here, it must be 
observed, the variation is not periodic. 

We must point out one more relation which exists 
between the atomic weights and a physical property of 
bodies—the power, namely, of emitting luminous rays. 
Tn fact, Lecoq de Boisbaudran has proved that for 


MENDELEJEFF’S PERIODIC LAW. - 169° 


analogous elements, such as potassium, rubidium, and 
cesium—calcium, strontium, and barium—aluminium, 
gallium, and indium, the increase of atomic weight is 
proportional to the increase of wave-length, which 
corresponds with the spectral lines of each of 
these metals. This profound idea was developed by 
the eminent chemist before the Chemical Society of 
Paris, but has not yet received sufficient publicity. We 
can only give the statement, observing that it has 
received a most striking confirmation from the dis- 
covery of gallium, and that it was possible to calculate 
very exactly the atomic weight of this metal, with 
those of aluminium and indium, from the position 
of the ‘corresponding’ spectral lines of these three 
metals. 

Such are some of the relations which exist between 
the atomic weights and the physical properties of 
simple bodies. Jt is an important chapter, to which 
Mendelejeff and Lothar Meyer have contributed many 
valuable developments; and amongst the theoretical 
consequences which follow from the conception of the 
Russian chemist we may mention the following: it 
contributes new elements to the classification of simple 
bodies, and controls views founded upon other consider- 
ations. A few remarks upon this subject will be 
necessary. 


170 THE ATOMIC THEORY. 


IV. 


In the table upon pp. 159, 160 the elements are ar- 
ranged in groups and series.. The vertical columns 
are composed of elements which resemble each other in 
the whole of their properties, and especially in the nature 
of their compounds. The elements are there arranged 
in families. If, with the increase of atomic weights, 
the properties of elementary bodies are subject to periodic 
variations, then those elements constituting a period may 
be placed in one group, and since in each of these groups 
the properties are subject to analogous variations, the 
corresponding terms of each group may also be con- 
nected: a certain concordance or harmony will be 
observed in these ‘harmonic’ or ‘homologous’ terms, 
which will form a serves. In some of these series, if not 
in all, we shall find in the increase of the atomic 
weights that regularity which was pointed out at the 
commencement of this chapter. The result of the pe- 
riodic law which considers the variations of properties 
in each group has therefore for a corollary a principle 
of classification or seriation which establishes analogies 
of properties in each family of analogous bodies. This 
is an important fact, and it is a circumstance worthy of 
remark that such varied and unexpected developments 
arise from the simple idea of arranging bodies according 
to the increasing value of their atomic weights. This 
simple idea was a most important one.’ 

The horizontal groups contain, as we have seen, 


1 It is right to observe that Mendelejeft’s idea is somewhat analo- 
gous to an idea long ago promulgated by De Chancourtois. 


MENDELEJEFF’S PERIODIC LAW. 171 


groups of elements in which the physical properties are 
subject to periodic variations. We must now proceed 
to show how their chemical properties are gradually 
modified from term to term. 

Tn the first place this is the case with the ‘ electro- 
chemical’ character of elements. The variations to 
which the electro-chemical properties are subject from 
one term to another, and which were given for the first 
and second groups upon p. 156, appear also in the 
other groups, except that in some of these groups the 
variations from the first term to the last pass through 
two periods instead of one. This, for example, is the 
case with the group which commences with potassium 
and for that which commences with rubidium. Thus 
after potassium we. have the electro-positive metal 
calcium, after which the electro-negative character 
appears. in titanium, vanadium, and chromium. With 
manganese and iron the electro-positive character is 
again seen, becoming more pronounced with nickel and 
cobalt. This is also the case with the group which 
commences with electro-positive rubidium, and is closed 
by palladium, also electro-positive. 

On the other hand, the group which commences with 
silver, an electro-positive metal, finishes with tellurium 
and iodine, both electro-negative. 

The electro-chemical character of elementary bodies 
exercises some influence upon their power of combina- 
tion with different elements. It is worthy of remark 
that those metals which are strongly electro-positive 
have great tendency to form with electro-negative 
oxygen the simple and generally stable compounds of 


172 THE ATOMIC THEORY. 


the protoxides. The higher degrees of oxidation are 
rare and unstable. The contrary is the case with the 
electro-negative metals and metalloids; here the 
degrees of oxidation are numerous, and the higher terms, 
very rich in oxygen, form powerful acids. 

Again, the nature of the compounds formed by 
the elements with electro-positive hydrogen increases in 
simplicity as the electro-negative character of the 
element becomes more pronounced. Hydrochloric, 
hydrobromic, and hydriodic acids may be quoted as 
examples. 

As a general rule, in considering the power of com- 
bination with a given element possessed by the simple 
bodies which form part of one group, we observe a regular 
gradation, to which Mendelejeff has called attention. 
Without repeating the facts mentioned on p. 157, we 
may here remark that the capacity of combination 
with oxygen possessed by simple bodies increases regu- 
larly with their atomic weight to a certain point, after 
which it begins to decrease again. That such is the 
case will be seen from the following table, which contains 
some oxygen compounds of the different elements. 
Observe that the vertical series here correspond with the 
horizontal series of the table given on pp. 159,160. It 
is also important to remark that, with a few exceptions, 
comprising the peroxides, the oxygen compounds quoted 
here are the richest known: they therefore show the 
limit of the capacity of combination with oxygen 
possessed by the elements. j 


173 


MENDELEJEFF’S PERIODIC LAW. 


Co’D 
°0°°L 
*o"qs 
*o*ag 
*o*"ul 
“0°PO 

O*SV 


pe 
209719 
o*att 


“ord 
7oun 
‘ony 
Coa) 
(°0%98) 
*O'sV 
ore 
‘ofuz 
ong 


‘spunoduoy uabhaeg fo saruvay 


O70) = 
208g P 
*0'd *0°N 
‘O"IS '0*0 

‘OV ‘o*d 
0° x Obs 3) 
O*eN (o*rT) 


174 THE ATOMIC THEORY. 


To make the progression in the power of combination 
of elements with oxygen clear, the formule of the oxides 
have been doubled in the second, fourth, and sixth 
horizontal series. This progression is at once seen upon 
glancing at the vertical series; but we also find that it 
attains a maximum at about the seventh or eighth 
term, after which we notice a diminution in the rich- 
ness of oxygen. It appears, therefore, that the capacity 
of combination with oxygen possessed by simple bodies 
forming part of a given group passes through varia- 
tions similar to those noticed above (p. 157) in con- 
nection with the compounds formed by the simple 
bodies of the first and second groups with oxygen, 
chlorine, and hydrogen. The periodic law is here again 
evident, as with the physical properties. 

We must, in conclusion, notice one more peculiarity 
referred to by Mendelejeff. 

The composition of the hydrates is naturally con- 
nected with that of the oxides. If, as is allowable, we 
regard the hydrates of the well-marked oxides as com- 
binations of simple bodies with OH (hydroxyl).groups, 
we observe that two hydroxyl groups correspond to each 
atom of oxygen in an oxide, thus— 


Ca’0 + H,O = Ca”(OH),. 


This notation is now in general use ; but Mendelejeff, 
who was one of the first to use it, remarks that the 
number of hydroxyl groups which a simple body 
has the power of fixing appears to be determined by the 
number of hydrogen atoms contained in its hydrogen 
compound, or again by the number of ethyl groups con- 


. 
oe a ae 


MENDELEJEFI’S PERIODIC LAW. 175 


tained in the ethyl compound. Thus, to take an 
example from the sodium group, the electro-negative 
elements of this series follow each other in the following 
order: silicon, phosphorus, sulphur, chlorine. Now, 
we find that their most stable hydrates contain as many 
hydroxyl groups as their hydrides contain atoms of 
hydrogen and their ethides ethyl groups. 


Hydrates. Hydrides. Ethylides. 
Si(OH), SiH, SiEt, 
PO(OH), PH, PEt, 
S0,(OH), SH, SEt, 
C10,(OH) ClH N1Et. 


These developments are sufficient, and we must 
conclude. It clearly results from the above that the 
most important physical properties and the fundamental 
chemical properties of simple bodies stand in some 
relation to their atomic weights: they are a func- 
tion of the atomic weights. This is the result of general 
investigation, and, in spite of the uncertainty which 
still reigns as tothe precise manner of this function, 
and notwithstanding some objections or imperfections 
of detail, we may say that the principle indicated by 
the Russian chemist will henceforth furnish one of the 
bases of chemical classification. Now, it is evident, in 
conclusion, that the relations in question would not 
appear, and the principle which connects them could 
not have been formulated without the adoption of the 
present system of atomic weights. These relations 
would have remained hidden or obscure had the attempt 
been made to deduce them from ‘equivalents.’ I lay 
stress upon this point of view, and remark, finally, 


176 THE ATOMIC THEORY. 


that the discoveries of the eminent Russian chemist 
furnish a strong argument in favour of the new system 
of atomic weights. Mendelejeff himself speaks very 
decidedly upon this point. These are his own words :— 
‘Our conceptions upon atomic weights have latterly 
acquired such solidity, especially since we have applied 
to them the law. of Avogadro and Ampére, and since 
the works of Laurent, Gerhardt, Regnault, Rose, and 
-Cannizzaro, that we may confidently affirm that the idea 
of atomic weights—that is to say, the smallest quantity 
of an element contained in a molecule of its combina~ 
tions—will continue without alteration through all the 
variations to which chemical theories may be subject.’ ! 


Vie 


We must now consider the new system of atomic 
weights as furnishing new elements in the classification 
of simple bodies. The principle of this classification 
will be that of the natural method: each group must be 
composed of bodies which resemble each other in their 
zhemical properties, in the nature, form, and functions 
of their principalcompounds. Simple bodies belonging 
to the same family or series form similar compounds, 
and the atomic weights attributed to these simple 
bodies should be such that the similar compounds may 
receive analogous formule. This side of the question 
has already been touched upon in the preceding 


1 Die periodische Glesetzmdssigheit der chemischen Elemente, yon 
D. Mendelejeff, p. 4; St. Petersburg, August 1871. 


NATURAL CLASSIFICATION. 177 


chapter, but it will be useful to return to it and to bring 
forward a few more proofs, drawn from purely chemical 
considerations, in favour of the system of atomic weights 
now generally adopted. 

Arsenic and antimony are two closely related ele- 
ments; their similar compounds should therefore re- 
ceive analogous formule. Gerhardt was right in halving 
the atomic weight which had previously been attributed 
to antimony, the chlorides, oxides, and sulphides of these 
two simple bodies thus receiving the formule— 


AsCl, SbCl, 
As,0, Sb,0, 
As,O, Sb,0; 
As,8, Sb,8,, 


which notation demonstrates the analogy of these com- 
pounds. | 

There is no difference of opinion upon this point, 
which, however, cannot be said for the double atomic 
weights which Cannizzaro has attributed to certain 
metals in order to make them agree with the law of 
Dulong and Petit, and also with the law of gaseous 
densities. 

We remarked above that Berzelius had represented 
all protoxides by RO, while Gerhardt attributed to them 
all the formula R,O. It is now admitted that there are 
two classes of protoxides. The first, R,O, are formed of 
two atoms of metal and one of oxygen; the second con- 
tain a single atom of metal and one of oxygen. Now, is 
such a distinction founded upon a chemical basis, and 
are we authorised in separating the alkaline metals, to 
which may be added silver and thallium, which all form 


| Ee: jaar THE ATOMIC THEORY. 


protoxides, R,O, from the metals of the alkaline earths 
and so many others forming protoxides ? 

This classification is proved to be quite legitimate 
from the special character of the alkaline metals, which 
form a number of isomorphous and characteristic com- 
pounds. 

Silver has been correctly classified with the alkaline 
metals. We may remind our readers of the isomorphism 
of anhydrous sodium sulphate and silver sulphate. We 
would also draw attention to the fact that in the alums 
and the double sulphates of the magnesian series, 
SO,R” + 50,M,’+6H,0O, potassium may be replaced by 
sodium and ammonium, but not by calcium or barium. 

Cream of tartar or acid potassium tartrate is easily 
saturated by sodium carbonate or by ammonia, and the 
mixed salts which are thus obtained—double potassium 
and sodium tartrate, and double potassium and am- 
monium tartrate—are as definite and stable as the 
neutral salt of potassium. If, on the contrary, cream 
of tartar is saturated with chalk, the result is a very 
unstable compound, which bears no resemblance to the 
sodio-potassium tartrate and others of that class. 

The following is a peculiarity of the same kind and 
equally characteristic. The alkaline metals, or rather 
the alkaline bases, have a marked tendency to form acid 
salts with the dibasic acids. ‘The acid sulphates, car- 
bonates, oxalates, and tartrates of potassium are very 
well defined, and relatively stable salts. The acid salts 
formed by the alkaline earths are, on the contrary, but 
few in number, and when we do meet with them they 
are very unstable and are decomposed by water. Thus 


NATURAL CLASSIFICATION. 179 


there is no acid oxalate of calcium, and the acid oxalate 
of barium, which has been described, is so unstable that 
it cannot be dissolved in water. 

It appears from these facts that the alkaline metals 
and the monovalent metals in general form a perfectly 
distinct group, and we shall find that the bivalent metals 
are ecually well characterised by certain properties. 
Two atoms of chlorine or two residues of monobasic acid | 
are required to saturatethem. They can fix at the same 
time two different monatomic elements or two residues 
of different acids, and their capacity of combination 
accounts for the existence of compounds analogous to 
the following :— 


Cl ak), C.H.0. Ha, 
He” a0 RU Sgye Es pte Bae tatad po’ 
I~ NO,7 NO,” Cl 
Mercury chloro- Strontinm Barium aceto- Plumbic aceto-chlor- 
iodide. aceto-nitrate, nitrate. hydrin. 


The argument derived from the existence of these 
mixed compounds in favour of the existence of bivalent 
metals, and, consequently, of the duplication of the 
atomic weights of these metals, is of the same order as 
that which was formerly drawn by Liebig from the con- 
stitution of the sodio-potassium tartrate in favour of 
the dibasicity of tartaric acid. 

This group of bivalent metals is further distinguished 
by the tendency which is shown by their oxides to form 
dibasic salts. This, as is well known, is especially the 
case with the oxides of copper and lead. 

The duplication of the atomic weights of calcium, 
magnesium, and lead enables us to represent in a very 
simple and striking manner the constitution of certain 


180- THE ATOMIC THEORY, 


minerals belonging to the wagnerite and apatite group 
(see p. 139). | 

Take, for example, the latter mineral. It is gene- 
rally represented as formed of 3 molecules of ordinary 
phosphate of lime and one molecule of calcium fluo- 
ride. This formula agrees with analysis, but it would 
be extremely difficult to demonstrate, on the dualistic 
theory of salts, the existence of such an unusual com- 
bination. If, however, we regard calcium as a bivalent 
metal, capable of replacing two atoms of hydrogen in 
phosphoric acid, we shall see that the presence of an 
atom of fluorine or chlorine is necessary to saturate the 
remaining affinities. Three molecules of ortho-phos- 
phoric acid, PO,H;, contain 9 atoms of hydrogen. If we 
add 4 atoms of calcium, 8 atoms of hydrogen only 
are displaced and the acid is nct saturated. If we add 
5 atoms of calcium, the presence of which is attested by 
analysis, there will be an excess of calcium, for the fifth 
atom of this metal, only finding one atom of hydrogen 
to displace, will not be saturated: the atom of fluorine 
comes in to complete the saturation. The following 
formule will explain the combination from this point of 


view :— 
{Cay Ca 
"i (PO,)3H, (PO,)s heist - (PO,)s (Ca FY’ 
3 molecules of phos- Calcium phosphate non- ’ Apatite. 
phoric acid. saturated. 


In a great number of other compounds chlorine plays 
the part taken by fluorine in wagnerite and apatite. 
This is the case in the combinations described by Carius 
under the name of plumbic aceto-chlorhydrin, aceto- 
bromhydrin, and aceto-iodhydrin (p. 179). 


NATURAL CLASSIFICATION. ‘181 


),H,O " 
‘tepa t 
Plumbic aceto-iodhydrin. 

I have drawn attention to one more argument in 
favour of the duplication of atomic weights, and con- 
sequently of the bivalency of certain metals. In 
Gerhardt’s notation the formule of a large number of 
hydrated salts had been so arranged that each molecule 
of the anhydrous salt contained a half-molecule or 
an uneven number of half-molecules of water. This 
inconvenience is removed if the atomic weights of the 
metals contained in these salts are doubled. { have 
given upon p. 463 of vol. i. of the ‘ Dictionnaire de 
Chimie pure et appliquée’ a number of examples in 
explanation of the argument in question; but I must 
confess that for two reasons I now attribute less im- 
portance to this argument. 

In the first place there are exceptions, for we find 
salts containing bivalent metals, a molecule of which 
cerystallises with a half-molecule or an uneven number 
of half-molecules of water, so that if we wished to 
represent the water of crystallisation by entire molecules, 
we should have to take two molecules of the anhydrous 
salt. 

In the seccnd place, we must not forget that the 
smallest quantity of a crystal is a different matter to 
the smallest quantity of a salt—that is to say, a molecule 
—and we may well imagine that in the formation of a 
crystal 2 molecules of a salt may unite with 1 molecule 
or with an uneven number of molecules of water. We 


shall, however, presently discuss the water of crystallisa- 
tion. 


182 THE ATOMIC THEORY. 


It appears from the above discussion, that chemical 
analogies, in agreement with the law of specific heats, 
the law of gaseous densities, and the law of isomorphism, 
authorise us in doubling the atomic weights of a great 
number of metals. The compounds of these metals 
receive, therefore, special formule, similar to those which 
were formerly given to them by Berzelius, and differing 
from those which we now attribute to the corresponding 
compounds of the alkaline metals. In fact, the com- 
pounds of the metals, whose atomic weights have been 
doubled, and which we call bivalent, may be placed, 
as regards their molecular complication, between the 
corresponding compounds of the alkaline metals and 
silver, and those formed by the sesquioxides. 

For example :— 


Oxides. Hydrates. Chlorides. Nitrates. Sulphates. 
K,O K(OH) KCl NO,K SO,K, 
Ag,O AgCl NO,Ag SO,Ag, 

Ca O Ca”(OH), Ca’Cl, (NO,),Ca”  SO,Ca” 

Pb 0 Pb”(OH), Pb’Cl, (NO,)Pb” SO,Pb” 
Sb,!0, Sb’”(OH), SbCl, (SO,),Sb,/” 
Bi,”0, Bi’(OH), Bi"Cl, (NO,),Bi” — (SO,),Bi,”” 


The formule of the sesquioxides and the corre- 
sponding compounds are universally admitted, but some 
chemists refuse to adopt the notation which expresses 
the diatomic nature of certain metals. It complicates, 
they say, the demonstration of the science. Certain 
formule would undoubtedly gain in simplicity if we 
adopted for the metals in question the halved atomic 
weights; but are we justified in misrepresenting reac- 
tions and ignoring the most evident analogies under 
the pretext of simplicity? The universal acceptance 


a = 


ees 


NATURAL CLASSIFICATION. 183 


of the notation in question, as regards organic com- 
pounds, shows emphatically how natural and correct it 
is as regards mineral compounds. ‘This point deserves 
explanation. 

We represent the action of hydriodic acid upon the 
hydrates of potassium and lead by the following equa- 


tions :— 
K(OH) + HI =KI +4H,0 
Pb’(OH), + 2HI = PbI, + 2H,0. 


1 il 


It would be simpler, they say, to halve the second by 
halving the atomic weight of lead,' and to write— 


Pv'(OH) 4 HI = PU'l + H,0. 


The latter would unquestionably be simpler, and 
strictly equivalent to the former. But it is an impor- 
tant fact that the formula Pb’(OH), and the equation 
in which it is given enable us to trace an interesting 
connection and demonstrate an evident analogy—that, ° 
namely, which exists between the hydrates of mineral 
chemistry and those of organic chemistry. These 
hydrates of potassium and lead correspond to the 
hydrates of ethyl and ethylene, and the action of hydri- 
odie acid upon the latter hydrates is represented by the 
following equations :— 


C.H,(OH) + HI = C,H,I + H,O 
Ethyl hydrate Ethyl 


(alcohol). iodide. 
C,H,’(OH), + 2HI = C,H,I, + 2H,0 

Ethylene d hydrate Ethylene 

(glycol). iodide. 


Who would now think of halving the latter under 


1 Pb = 2064; Pb’ = 103-2. 


184 THE ATOMIC THEORY. 


the pretext of rendering it more simple and exactly 
comparable to the first equation ? 

Berthelot himself, who upheld this antiquated 
argument, could not consent to write— 

CH,(OH) + HI = CH,I + H,O 
Ethylene Ethylene 
hydrate. iodide. 

It is generally admitted by chemists that the 
ethylene compounds should, to a certain extent, 
rank between the ethyl compounds and the glycerine 
compounds, and that the series to which they belong 
increase in complication, as may be seen from the 
mineral compounds given in the preceding table. We 
have a curious connection and an evident parallelism 
between the mineral and organic oxides, hydrates, 
chlorides, and salts. 


Oxides. Hydrates. Chlorides. Ace‘ates. 
(C,H,’),0 C,H,’.OH C,H, ’.Cl C,H,'.C,H,0, 
Ethyl oxide. Ethyl hydrate. Ethyl chloride. Ethyl acetate. 
(G,H,")0 _0,H,"(OH), ©” (CHC, = CHC 
Ethylene oxide. Ethylene hydrate. Ethylene chloride. Ethylene acetate 
(glycol). 


C,H,’"( OH), C.H,”.Cl, C,H,”’( C,H;0,)g 
Glyceryl hydrate Glyceryl chloride Glyceryl acetate 


(glycerine). (trichlorhydrin). (triacetin). 
C,H."( H), C,H,'".Cl, C,H,'"(C,H;0,), 
Erythrite. Erythryl chloride. Erythryl acetate. 


It is also worthy of remark that the reactions which 
produce these compounds or by which they are trans- 
formed also show a regular gradation and increasing 
complication. Take, for example, the action of potash 
upon the mineral salts and upon the ethers we have just 
mentioned. It brings into play one, two, or three 


= 
Se 


NATURAL CLASSIFICATION. 185 


molecules of alkali, and produces mineral and organic 

hydrates, which are exactly comparable to each other, as 

regards the degree of hydration. Thus, potash reacts 

in the following manner upon the metallic nitrates :— 
NO,Ag + KOH = NO,K +. AgOH! 


(NO,),Pb” + 2KOH = 2NO,K + Pb’(OH), 
(NO,),Bi” + 3KOH = 3NO,K +Bi”(OH),. 


The second reaction naturally takes its place between 
the first and the third reactions. 

In organic chemistry we meet with a similar gra- 
dation. When potash reacts upon the ethers, one, two, 
or three molecules of alkali take part in the reaction, as 
in the preceding case, according to the more or less 
complex nature of the ether :— 


C,H,0,.C,H,’ + KOH = C,H,0,.K + 0,H,/(OH) 


Ethyl acetate. Potassium acetate. Alcohol. 
(C,H,0,).C,H,” + 2KOH = 2(C,H,0,.K) + C,H,’(OH), 
Ethylene acetate. Potassium acetate. Ethylene dihydrate 

(glycol). 
(C,H,0,),(C,H,’”) + 3KOH = 3(0,H,0,.K) + C,H,’”(OH), 
Glyceryl] triacetate Potassium acetate. Glyceryl trihydrate 
(triacetin). (glycerine). 


In these mineral and organic hydrates we see that 
the number of the groups (OH) which marks their 
degree of hydration increases regularly, just as the 
atoms of chlorine increase in the corresponding chlorides. 
Again, the chlorides and hydrates of the second class are 
necessarily and naturally intermediary between the first 
and third class. It is, therefore, an incontestable fact 
that these compounds, and the reactions which give rise 


1 Since silver hydrate does not exist, the equation must be written— 
2NO0,Ag + 2KOH =2NO,K + Ag,0 + H,0. 
9 


186 THE ATOMIC THEORY. 


to or modify them, differ in their form. Chemistry 
would be a very simple science if this form were the 
same for all compounds, and if all reactions were, to 
some extent, cast in the same mould. The system of 
atomic weights, which is now generally adopted, shows 
that there is a regular gradation in these forms of com- 
bination and in these reactions, and brings to light the 
harmony which exists between the reactions of mineral 
chemistry and those of organic chemistry. It is the 
last argument which we add to all those which we have 
already advanced in favour of this system of atomic 
weights. . 


es or - 


CHAPTER VII. 


ATOMIC AND MOLECULAR VOLUMES, 


By the term atomic volumes of simple bodies is 
meant the volumes occupied by quantities of these 
bodies proportional to their atomic weights, and by the 
term molecular volumes of compound bodies, the vo- 
lumes occupied by quantities of these bodies proportional 
to their molecular weights. 

To determine the relative volumes occupied by atoms, 
we have only to divide the atomic weights by the weights 
of the unit of volume—that is to say, by the densities. 
The atomic volumes are the quotients of the atomic 
weights by the densities; the molecular volumes, the 
quotients of the molecular weights by the densities. 

If matter were continuous, these quotients would 
give the true volumes occupied by atoms relatively to 
the volume of one of them taken as unity. But this is 
not the case. 

The ultimate particles of bodies do not touch each 
other; they are separated by relatively large spaces. 
They move in ether, and in gaseous bodies their distance 
apart is immense in proportion to their size: it is very 


188 THE ATOMIC THEORY. 


considerable in solid and liquid bodies. The space 
occupied by the unit of volume of bodies is therefore far 
from being filled by the atomic substance itself; it com- 
prises a portion of ether probably considerable. In 
other words, the conception of the density of bodies 
comprises two distinct but inseparable elements— 
namely, the ultimate particles which we term atoms or 
molecules, and the interatomic or intermolecular spaces. 
This remark will show the exact meaning which must 
be attached to the expressions ‘atomic volumes’ and 
‘molecular volumes.’ 

If the molecules were situated at equal distances in 
the different bodies, it is clear that a given volume of 
the latter would contain the same number of molecules ; 
the molecular weights would be proportional to the 
densities and the molecular volumes uniform. This is 
the case with the gases. We admit that they do 
perceptibly contain, in a given volume, the same num- 
ber of molecules ; the relative weights of the latter are 
proportional to the densities. But it is different with 
solid and liquid bodies. Their molecules are situated 
_at various distances, not only in different substances, 
but sometimes in the same body. Thus their coefficients 
of expansion are very different, and, moreover, vary for 
a given body, according to the temperature and physical 
condition of that body. This unequal distribution of 
molecules in solid and liquid bodies makes it impossible 
to discover a simple relation between the molecular 
weights and the density, like that which we have just 
mentioned in connection with gaseous bodies. 

As regards liquid and solid elements, we know that 


ATOMIC VOLUMES. 189 


very wide limits must be assigned to the variations of 
their densities. 

The lightest of metals, lithium, has a density of 0°59 
and weighs 39 times less than the same volume of ham- 
mer-hardened platinum, the density of which is 23. 
These densities, moreover, vary according to the physical 
condition of the body, so that it is impossible to com- 
pare the densities of liquid and solid bodies, of amorphous 
and erystallised bodies, of bedies solidified after fusion 
and bodies beaten and hammer-hardened after solidifica- 
tion. In order to draw any comparisons from the atomic 
volumes of simple bodies and the molecular volumes of 
compound bodies, we must, therefore, calculate the 
densities under similar conditions—namely, for liquid 
bodies, at equal distances from their points of ebullition, 
as Hermann Kopp recommends ; and for solid bodies, as 
much as possible at equal distances from their points of 
fusion. 

We will now proceed to give a brief account of the 
result of this work and of all the facts which have been 
collected with regard to the relative volumes occupied 
by atoms and molecules. We shall confine ourselves to 
general results, referring our readers to special works for 
numerical data and details. 

The limits within which the atomic volumes of 
simple bodies vary are less considerable than in the case 
of densities, though still very wide. Mendelejeff has 
shown that these variations are a periodic function of 
their atomic weights; for if the elements are arranged 
in the order of the progression of their atomic weights, 
their atomic volumes increase and decrease periodically, 


190 THE ATOMIC THEORY. 


We have discussed this point at some length, and will 
not, therefore, return to it. We will only add that the 
numerical values of the atomic volumes of simple bodies 
will be found in the table given upon pp. 159, 160. 

It appears from these facts that there must be a 
relation between atomic weights and atomic volumes. 
Of the precise nature of this relation we are, however, 
ignorant. 

Dumas has remarked that certain simple bodies 
belonging to the same family have almost the same 
atomic volumes. This is the case with the following 
bodies :— | 


| Atomic Volumes | Atomic Volumes | Atomic Volumes 
Chlorine 25-6 | Sulphur 15-7 Phosphorus 13°5 
Bromine 26'9 Selenium 169 


Iodine 25°6 
| | | Tellurium 20°5 Antimony 18-2 


| 
Arsenic 13°2 
Bismuth 21:1 | 

We see that tellurium, antimony, and bismuth only 
partially conform to this rule; the following elements 
break through it entirely :— 


Atomic Volumes Atomic Volumes Atomic Volumes 
Carbon 3°6 Lithium 11°9 Calcium 25-4 
Silicon 11:2 Sodium 27°3 Strontium 34°9 
Zirconium 21°7 Potassium 45°4 Barium  36°5 

Rubidium 56:1 


We owe all our accurate information upon the 
molecular volumes of compound bodies to the extensive 
researches of Hermann Kopp, who devoted his attention 


MOLECULAR VOLUMES. 19i 


principally to the molecular volumes of liquid bodies. 
The results of these researches may be summed up in 
the following propositions, which apply especially to 
organic liquids. 

1. The molecular volume of compounds is expressed 
by the sum of the atomic volumes occupied by the 
elements. 

2. In compounds possessing a similar atomic com- 
position, the same element always possesses the same 
atomic volume. The latter being determined for every 
simple body, it follows that the molecular volume of a 
compound may be calculated if the atomic composi- 
tion is known. 

3. In compounds possessing different atomic struc- 
tures the same element may occupy two different 
volumes. Thus, to borrow an expression from the 
theory of types, the atomic volume of oxygen differs 
with its position either as contained in a radical, or 
situated without that radical, in the state of typical 
oxygen. Nitrogen possesses a different atomic volume, 
according as it is contained ina compound derived from 
the ammonia type, combined with carbon as in cyanogen, 
or united to oxygen as in nitrous vapour. 

Hermann Kopp succeeded in determining the atomic 
volumes of carbon, hydrogen, oxygen, nitrogen, &e., by 
means of ingenious considerations which we shall briefly 
describe as follows. 

1. In comparing the molecular volumes of organic 
compounds, which differed from each other only by 
mCH,, he found that for each addition of CH, the 
average increase of the volume of the molecule was 22. 


192 THE ATOMIC THEORY. 


We may therefore conclude that this number expresses 
the volume of one atom of carbon and two of hydrogen 
—that is to say, the volume of CH,. 

2. Two organic compounds which differ from each 
other by the addition of mC and the loss of nH, possess 
the same molecular volume. We may, _ therefore, 
conclude that C occupies the same volume in these com- 
pounds as H,, and as the molecular volume of CH, is 22, 
it follows that the atomic volume of carbon is 11 and 
that of H is 14 =5°5. 

3. The molecular volume of water at boiling point 
is 18°8 (instead of 18). If we subtract 11, the volume 
of H,, we have 7:8 for the atomic volume of oxygen. 
According to Hermann Kopp, oxygen only occupies this 
volume in organic compounds when it is contained in a 
typical residue, to use the expressions of that time—that 
is to say, when it is connected with two different 
atoms which it unites, as, for instance, the two atoms 
of hydrogen in water. It occupies a different volume 
when it is contained in a radical—that is to say, combined 
by its two points of saturation to the same atom of 
carbon as in aldehyde and acetone.! 

Aldehydecontaining C,H,O—that is to say, 2CH, + O 
—the volume which is here occupied by oxygen may be 
found by subtracting from the molecular volume of 
aldehyde (56 to 56:9) that of 2CH,=44. We thus 
obtain 12 to 12°9 as the atomic volume of oxygen 
when contained in an organic radical. 

Hermann Kopp adopts the mean 12-2. 


1 The two forms of oxygen compounds are given in the follow- 
ing table, which will explain the Cistinction in question :— 


MOLECULAR VOLUMES. 193 


Having thus calculated, by means of the above con- 
siderations, the volumes occupied in organic compounds 
by the atoms of carbon, hydrogen, and oxygen, the 
eminent chemist was able to calculate @ priori the 
molecular volumes of a number of ternary organic com- 
pounds, by adding together the sum of the atomic 
volumes of the elements, in accordance with one of the 
propositions given above. The molecular volume of a 
compound containing @ atoms of carbon, b atoms of 
hydrogen, ¢ atoms of oxygen in the radical, and d atoms 
of typical oxygen, may therefore be given by the 
formula— 


COMPOUNDS CONTAINING TYPICAL OXYGEN. 


Typical Formule. 
Methyl Methyl Ethyl Ethyl Ethylene 
Water. hydrate. oxide. hydrate. oxide. oxide, 


H CH, \ CH,) C,H, C,H 
SER Wt as GEO 3) 2Hs,9%, Ory pO, Cy 


Constitutional Formule. 


CH, CH, OH,CH, CH,CH, CH, 
| | | | | 
H—O—H O O O GO O 
| | | | < 

H CH, 4H CH,—CH, \CH,. 


COMPOUNDS CONTAINING OXYGEN IN THE RADICAL. 


Typical Formule. 


Aldehyde. Acetone. 
C,H,0 C,H,O 
H. CH, 
Constitutional Formule. 
OHy CH, 
| | 
CHO CO 


194 THE ATOMIC THEORY. 


MV =all + 05:5 +c12:2 + d7°8. 


The molecular volumes calculated in this manner 
have been compared with those deduced from experi- 
ment, when the molecular weights are divided by the 
densities taken at boiling point. The agreement be- 
tween calculated and experimental results in a great 
number of cases is sufficient to justify a serious con- 
sideration of Hermann Kopp’s conclusions. 

Omitting the consideration of the facts relative to 
the atomic volumes of other elements, such as sulphur, 
nitrogen, chlorine, bromine, and iodine, indirectly 
deduced from the molecular volumes of liquid com- 
pounds containing these elements, by means of pro- 
cesses similar to those just discussed, we must add a 
few words upon the molecular volumes of solid bodies. 
We must here confine ourselves to a few results regard- 
ing certain bodies endowed with a similar constitution 
and obtained under the same physical conditions. 

It is found that a great number of isomorphous 
bodies have the same molecular volume. This is the 
case with the sulphates of the magnesian series 
SO,M” + 7H,0, with the double sulphates of the mag- 
nesian series SO,M”’+S0,R,+6H,O, and with the 
alums. It seems, however, impossible to calculate the 
molecular volumes of solid compounds by means of the 
atomic volumes from the principles laid down for liquid 
bodies. Here the data of the problem are different. 
In proof of this we may, in conclusion, refer our readers 
to the relations pointed out by Playfair and Joule 
between the molecular volumes of certain crystallised 


MOLECULAR VOLUMES OF SALTS. 195 


salts and that of the water which they contain. We 
should suppose that the molecular volume of the crystal- 
lised salt would be equal to the sum of the volumes of 
the anhydrous salt and the water. But it is not so. In 
certain salts rich in water of crystallisation, such as the 
arsenates and phosphates which contain 12 molecules, and 
in the crystals of carbonate of soda which contain 10, the 
volume of this water (taken as solid) is equal to the 
volume of the molecule of the crystallised salt, the mole- 
cules of the anhydrous salts being as it were interposed 
between the molecules of water, without augmenting 
the volume of the latter. 


196 THE ATOMIC THEORY 


BOOK I 


ATOMICITY; 
OR VALENCY OF ATOMS IN COMBINATION. 


CHAPTER I. 


DEFINITION AND HISTORIC DEVELOPMENT OF THE IDEA OF 
ATOMICITY. 


In the preceding pages we have traced the origin and 
foundation of the atomic theory. We have seen this 
simple and correct idea which was brought forward 
by Dalton—namely, that the invariable proportions 
in which bodies combine represent the relative 
weights of their ultimate particles—gradually gain 
ground in science. We have explained the principles 
upon which the determination of these weights rests, 
as well as the physical laws by means of which these 
determinations are guided and controlled, thus render- 
ing to the hypothesis of atoms, which belongs to 


DEFINITION OF ATOMICITY. 19¥ 


the department of chemistry, assistance all the more 
unexpected and efficacious as coming from a different 
department of science. In concluding our explanation 
of the present system of atomic weights, we met with 
the idea that the ultimate particles of bodies, which we 
call atoms, do not all possess the same combining value : 
we saw that, while one atom of potassium unites with 
one atom of chlorine to form a chloride, an atom of lead 
takes two atoms of chlorine, and an atom of antimony 
three or even five. This difference in the power pos- 
sessed by simple bodies of forming more or less complex 
combinations with another simple body must be con- 
sidered as a peculiar property, inherent in their ultimate 
particles, and in order to distinguish it from affinity, 
which implies the force of combination, it has been 
termed atomicity, which is synonymous with combining 
value or valency of atoms. We must now show how 
this idea was first introduced into science, the precise 
sense in which it must be accepted, and the consequences 
resulting from it which affect chemical theories. 

These are fresh facts which give rise to fresh ideas. 
And the facts which are connected with the idea in 
question may be givenas follows in their historical order, 
the unequal saturating capacity possessed by bases for 
acids, and the unequal saturating capacity possessed by 
acids for bases. 

The first group of facts were long unknown. Berze- 
lius refused to admit the existence of sesquioxides, 
capable of saturating 3 molecules of acid, when prot- 
oxides could only saturate one. Gay-Lussac attri- 
buted to these sesquioxides a constitution strictly 


198 THE ATOMIC THEORY. 


equivalent to that of the protoxides, so that one mole- 
cule of oxide (an equivalent) should saturate one mole- 
cule (an equivalent) of acid. Some time afterwards, 
when the existence of polyacid bases was admitted, 
though set aside with other facts whose theoretical 
signification was not considered, Graham discovered 
polybasic acids. This discovery created a sensation 
and caused some difficulty in the conception and defini- 
tion of equivalent quantities (see p. 76). Neverthe- 
less fifteen years passed before the significance of this 
fact was recognised from the point of view now occupy- 
ing our attention. 

In the memorable work which was published in 1851 
upon etherification, and which marks a new era in the 
history of chemical doctrines, Williamson, generalising 
an idea first published by Laurent and Sterry Hunt, 
brought forward the proposition that a great number of 
organic and mineral compounds may be referred to the 
type of water. He held that such was the case with 
monobasic acids and with the salts derived from them. 
Acetic acid, for example, and potassium acetate were 
represented by the formulee— 


C,H, C,H,O 
pO pO 
which are constructed upon the model of that of 
water— 
H 
itp O 


the radical C,H,O0 and potassium, K, taking the place 
of an atom of hydrogen. But the eminent English 


~~ 


ATOMICITY. 199 


chemist also perceived that polybasic acids, which are 
not equivalent to the monobasic, present a greater 
molecular complication, and should be referred to a con- 
densed water type. Thus dibasic sulphuric acid was 
regarded as being derived from 2 molecules of water, 


H 
Hi} 


by the substitution of the radical SO, for two atoms 
of hydrogen. The formula of sulphuric acid becomes 


therefore, 
SO, 
wy 


and the radical SO, here takes the place of two atoms 
of hydrogen. Williamson has written this in two lines, 
and how productive of developments has this idea been 
which was announced with such simplicity. Odling, by 
an ingenious notation which is still in use, first marked 
this difference in the capacity for saturation possessed by 
the acetyl and sulphuryl radicals, by giving to their 
formule a different index— 


(C,H;0)’ | (SO,)” 
rae Hp it, pO» 
Acetic acid. Sulphuric acid. 


The idea that the substituting value of sulphuryl is 
twice that of acetyl is clearly expressed in this notation. 
We here find the germ of the modern theory of radicals 
which underwent such important developments a few 
years later, and which has superseded the old concep- 
tions of Lavoisier, Berzelius, and Liebig. This theory is 
still accepted, as well as the notation by which it is 


200 THE ATOMIC THEORY. 


perpetuated and represented,’ though it is now subordi- 
nate to a more general theory of which it appears as the 
natural consequence. We must now trace this further 
development. 

In the first place we should notice that, on the oc- 
casion of his fine researches upon the organo-metallic 
compounds, Frankland called attention, in 1872, to the 
power which metals possess of combining with a fixed 
and definite number of atoms. This idea, which was 
then new, formed the starting point of the theory of the 
saturation of elements and contains the germ of the 
theory of atomicity. 

In a note upon the theory of glycerine compounds? 
the author showed that glycerine may be regarded as a 
hydrate of the radical C,H, and its composition repre- 


sented by the formula (Ushi) | Os, which is similar to 
3 
the formulz by which, in accordance with the ideas of 
Williamson, ordinary phosphoric acid, cae Oss is re- 
3 


presented. In fact, the several series of glyceric ethers 
are comparable to the several series of ordinary phos- 
phates or orthophosphates.* This radical, (C,H,)’”, which 
can replace 3 atoms of hydrogen, is formed by the sub- 
traction of 3 atoms of hydrogen from the saturated 
hydrocarbon C,H,. Starting from this fact, which was 
then acknowledged as true, and has not since been 


1 The formule C,H,0.0H and SO,(OH),, now in general use, are 
only a variation of those used by Williamson. 

2 Ann. de Chimie et de Phys., 8° sér. t. xliii. p. 492. 

8 And not, as was stated by Berthelot in his remarkable memoir, 
to the phosphates, pyrophosphates, and metaphosphates 


ATOMICITY. 201 


invalidated by any fresh facts, that those hydrocarbons 
which are richest in hydrogen belong to the series 
C,H,,42, among which we find the hydrocarbon C,H, 
(propane), the author has derived the radical C,H, from 
that hydrocarbon by the subtraction of 3 atoms of hy- 
drogen. He proves that the radical C,H,, which can 
replace one atom of hydrogen, comes from the same 
hydrocarbon by the loss of a single atom of hydrogen. 
The subtraction of an atom of hydrogen developes a 
force in this residue C,H,, in virtue of which it is im- 
pelled to combine again with this hydrogen atom of 
which it has been deprived, or with some equivalent to 
it, and, on the other hand, this same force makes it ready 
to supply the place of an atom of hydrogen wherever it 
is wanting. Again, the loss of three atoms of hydrogen 
creates in the residue C,H,=C,H.—H, a force by which 
it isready to replace three atoms of hydrogen. Glycerine 
is produced in this manner, by the substitution of such 
a radical for three atoms of hydrogen in the type of 
three condensed molecules of water. 


Ay) 


; (C H y" 
H; J ®, ‘ a, O; 


Glycerine. 


The author has even gone further. He supposed 
that the five atoms of hydrogen were divided in the 
following manner among the three atoms of carbon 
[C,H,=CH,—CH-@H, |, which to my knowledge is the 
first attempt that was made at such a distribution of 
atoms ina radical. It resulted, however, in nothing, 
being a simple supposition. It was some years after- 
wards that Kekulé showed the ruling principle by which 


202 THE ATOMIC THEORY. 


such distributions of atoms may be conducted in a 
rational manner. 

There was a gap between the ‘ monobasic’ radical 
(C,H,)’ and the ‘tribasic’ radical (C,H,)’”, which the 
author was discussing. The residue C,H, obtained by 
the subtraction of two atoms of hydrogen from the hydro- 
carbon C,H,, should possess a substituting or combining 
value equivalent to these two atoms of hydrogen. This 
proved to be the case from the study of Dutch liquid and 
its analogues, which resulted in the discovery of the 
glycols. This residue or radical C,H, is propylene, and 
can replace, like its homologue ethylene, two atoms of 
hydrogen in two condensed molecules of water. The 
bodies possessing this constitution are the glycols. 


H CE’ (OS Sy 
jo CAR} COB 
Glycol. Propy! glycol. 


These ‘ diatomic’ radicals, as from that time 
they have been called, can also combine directly with 
two atoms of chlorine or bromine, as the Dutch che- 
mists showed at the end of the last century. The author 
has remarked that the phenomena belong to the same 
class as those presented by the direct combination of a 
metal with chlorine or bromine. 

Thus the substituting value marks the combining 
value. There is a connection between the two, and 
ethylene, which can replace 2 atoms of hydrogen, can 
combine directly with 2 atoms of chlorine or bromine, 
or, again, with 2 atoms of hydrogen (Berthelot) or their 
equivalent. In the same manner the radical sulphuryl 
(SO,)”, which can replace 2 atoms of hydrogen in 2 


ATOMICITY. 203 


condensed molecules of water (p. 199), can also com- 
bine with 2 atoms of chlorine to form  sulphuryl 
chloride (SO, )’Cl, (Regnault). The radicals of mineral 
and organic chemistry possess, therefore, as far as their 
combining or substituting value is concerned, all the 
attributes of simple bodies. This substituting value 
of radicals, correlative with the combining capacity, 
has received a definite name: it was then, and is 
still, called ‘atomicity.’ We shall soon extend it to the 
elements themselves (p. 211), of which the radicals, 
as just defined, are but in a measure the representa- 
tives. 

I believe it is to Odling that the credit is due of 
having been the first clearly to enunciate the idea that 
the substituting or combining value of simple bodies is 
not the same. He attributed to ferric hydrate the 


reese 


formula a lO caustic potash being represented by 
3 


the formula re In the hydrate of sesquioxide of 


iron the metal therefore replaces 3 atoms of hydrogen, 
while potassium in caustic potash only replaces one 


atom of hydrogen. 
H Ky 
+0 Hy 0 
Water. Potash. 
H Fe” 
ol a en a 


3 molecules of water. Ferric hydrate. 


In his memoir upon radicals! the author has givena 


1 Ann. de Chim. et de Phys. (3), t. xlvi. p. 307. The author even 
proposed the hypothesis that the phosphorus in some manner en- 


(204 THE ATOMIC THEORY. 


similar formula to phosphorous acid, which he repre- 


dy 
sented as H 


3 
Nitrogen has been represented as a tribasic element 


derived from the type of three condensed molecules of 
hydrogen. 


Gi \ He NN” z) 
He N 4 1 BN fe 
Hydrogen. Ammonia. Nitrogen. 


- Oxygen and sulphur, the ‘ dibasic’ character of which 
was demonstrated by Kekulé, were regarded as derived 
from a condensed type of 2 molecules of water. 


H,\ H, 0” 7 
He J QO” oO” vf 
Hydrogen. Water. Free oxygen. 


croached upon the 3 molecules of water, as if an atom of phosphorus, 
P, were formed of 3 sub-atoms p,=P, each of which would replace 
an atom of hydrogen ina molecule of water, the residues of the 
3 molecules of water, each of which would have lost an atom of 
hydrogen, being thus saturated by the t7ibasic phosphorus. 


H p 
ae Ayo 
H p 
Hy 0 Hr yO 
H\ Pp 
Hage Hype 
3 molecules of water. Phosphorous acid, 


The idea that the atom of triatomic phosphorus is formed by the 
union of 3 sub-atoms, has been variously developed. It was aban- 
doned by the author because he found that in pentachloride of phos- 
phorus and phosphoric acid the atom of phosphorus must be divided 
into 5 sub-atoms. The idea of types served as a basis for this idea, 
but we see at once how much it contributed towards showing that 
elements have different substituting and combining values, and 
consequently that their atoms are not mutually equivalent. 


ATOMICITY. 205 


Chlorine, on the contrary, and the elements of that 
class, were, after Gerhardt, referred to the hydrogen 


type. 
a H H Gis) 
HY cy Cl f 


Hydrogen. Hydrochloric Free chlorine. 
acid. 


Thus the elements which we have just mentioned 
were regarded as differing from each other in their sub- 
stituting value, phosphorus and nitrogen replacing or 
combining with 3 atoms of hydrogen; oxygen and sul- 
phur replacing or combining with 2 atoms of hydrogen ; 
while chlorine could only unite with, or replace, a single 
atom. 


If. 


The idea that hydrochloric acid was derived from 2 
volumes of hydrogen by the substitution of 1 volume of 
chlorine for 1 volume of hydrogen, or, again, that it was 
derived from 2 volumes of chlorine by the substitution 
of 1 volume of hydrogen for 1 volume of chlorine, was 
fundamentally a very oldone. Dumas had remarked as 
early as 1828 that in the combination of 1 volume of 
chlorine with 1 volume of hydrogen, a combination 
which produces 2 volumes of hydrochloric acid, the 
atoms of hydrogen and the atoms of chlorine seemed to 
be halved.’ The idea was perfectly correct, though 
stated in words which made it erroneous. If Dumas 


1 Traité de Chimie appliquée aux Arts, t. i., Introduction, p. 
xxxviii. Berzelius in his treatise energetically protests against this 
idea of Dumas, which would have led the great French chemist, 


206 THE ATOMIC THEORY. 


had taken 2 volumes of hydrogen and chlorine instead 
of 1 volume, and spoken of molecules instead of atoms 
divided into half-atoms, he would have given to his pro- 
position a definite form, which may be clearly expressed 
by the following formule : — 


ih epee SL ae EN el 
H Oy) FS ee1 Cl 
2 vol. 2 vol. 2 vol. 2 vol. 
(1 molecule) (1 molecule) (1 molecule) (1 molecule) 
of hydrogen. of chlorine. of hydrochloric acid. 


Under any circumstances it is evident that the import- 
ant distinction which we find in Dumas’s conception 
between two species of ultimate particles, atoms and 
half-atoms (which we now call molecules and atoms), 
appears again in science long after the ideas of Avogadro 
and Ampére had sunk into oblivion. Free hydrogen 


had it been adopted and developed, to a truer conception of the 
theory of volumes than that which satisfied the learned Swede. 
This conception of Dumas is, I think, so important, that the passage 
in which he states it should be given in his own words. 

‘These considerations are so simple that it is needless to dwell 
longer upon them. With the application, however, comes the diffi- 
culty. Takea litre of chlorine, and‘let us suppose it to contain 1,000 
atoms; alitre of hydrochloric acid should contain the same number. 
Now, 

1 litre of hydrogen = 1,000 atoms 
and 1 litre of chlorine = 1,000 atoms 
forming 2 litres of hydrochloric acid = 2,000 atoms. 


But each atom of chlorine upon combining with one atom of hydro- 
gen can only produce one atom of hydrochloric acid, or 1,009 atoms 
inall, We must, then, admit that the atoms of chlorineand hydrogen 
are halved in order to form the atoms of hydrochloric acid. Each 
of the latter are therefore composed of a half-atom of hydrogen 
and ahalf-atom of chlorine. This is also the case with deutoxide of 
nitrogen.’ | 


ATOMICITY. 207 


and chlorine are formed of two atoms combined with 
each other. The latter feature is important, and was 
added by Gerhardt, who expressed the same thought in 
these words: free chlorine is a chloride of chlorine, free 
hydrogen a hydride of hydrogen. Such was the new 
idea which was to make its way into science. To repre- 
sent the molecules of diatomic gases as composed of two 
atoms combined with each other, was to admit that 
these atoms have a mutual affinity, similar to that which 
unites the dissimilar atoms of compounds ; to regard the 
hydrogen molecule as belonging to the same class of 
combination as hydrochloric acid; to represent the 
direct combination of chlorine with hydrogen as a double 
decomposition ; and to restore, in a word, though in a 
simpler form, the proposition of Avogadro and Ampére 
and the beautiful conception of Dumas. 

Thus hydrogen, chlorine, oxygen, and nitrogen are 
formed, in a free state, of 2 atoms combined with each 
other. This proposition is supported by a number of 
chemical proofs. 

It is well known that even finely divided copper is 
scarcely attacked by hydrochloric acid at ordinary 
temperatures. Hydride of copper, on the contrary, is 
attacked by it with great energy. Brodie was the first 
to observe that this reaction was easily explained if, in 
addition to the affinity of chlorine for copper, the affi- 
nity of hydrogen for hydrogen was admitted. 


— + —+ 
1CuH +HCl=CuCl+HH. 
As regards oxygen, the conception in question has 
1 Cu=63. 


Y 
~ 


208 THE ATOMIC THEORY. 


received startling confirmation from the celebrated ex- 
periments of Brodie upon the reciprocal action of 
peroxides. The fact that peroxide of hydrogen reduces 
oxide of silver, permanganic acid, and perchromic acid 
so easily, and at the same time is itself reduced with a 
brisk liberation of oxygen, was formerly explained as the 
action of contact, an expression which means nothing. 
Brodie ascribed it to the natural play of affinities. The 
oxygen which is added to water in hydrogen peroxide 
unites with the oxygen of the oxide of silver or with 
the excess of oxygen in the highly oxidised acids, and 
one atom of oxygen uniting with another forms a mole- 
cule of oxygen which contains both atoms and is evolved. 
This affinity of oxygen for oxygen is stronger than that of 
water for oxygen and than that of peroxide of manganese 
for oxygen. This is why bodies saturated with oxygen 
can reduce each other, without a combination taking 
place between the products of this reduction. 

Another class of arguments may be brought forward 
in support of this important idea. The peculiar 
activity of hydrogen and oxygen when wm the na- 
scent state is undoubtedly due to the fact that under 
these circumstances the atoms act separately, before they 
have been united to another atom to form the pairs of 
which the molecules are composed. It is evident that 
heat should be disengaged by this formation, which is a 
combination. The isolated atoms which are just formed 
and not yet united into pairs are still provided with this 
heat, and have, consequently, the greater activity.' 


1 This idea was published long ago by P. A. Favre (Comptes 
Rendus, t. xiii. p. 369). 


eee a 


ATOMICITY. 209 


Berthelot has uselessly brought forward in opposition 
to this conception considerations drawn from the thermal 
phenomena which accompany the formation of the un- 
stable combinations just mentioned Hydride of copper, 
he said, was undoubtedly formed with absorption of heat : 
it is not astonishing therefore that it should be attacked 
in the cold by hydrochloric acid, when copper is not. 
The argument seems to rest upon giving a natural expla- 
nation of thereaction in question. But some reserves must 
be made upon the premises. What meaning must be 
attached to the proposition, hydride of copper is formed 
with absorption of heat? No chemical combination can 
give rise, as a combination, to an absorption of heat, for 
the connection and fixation of the ultimate particles cf 
bodies in new positions of equilibrium gives rise toa loss 
of energy, and consequently to a disengagement of heat. 
It is, however, possible for this action to be preceded 
or accompanied by an inverse action—that is to say, by a 
separation of the ultimate particles, a phenomenon which 
gives rise to an absorption of heat. These two actions, 
upon superposition, give rise to a result sometimes posi- 
tive, sometimes negative, according to their respective 
intensity. We cannot say, therefore, that copper and 
hydrogen have absorbed in the act of combining a cer- 
tain quantity of heat: they have, on the contrary, libe- 
rated heat. But, while separating from the combination 
which contained them in the first place,! the elements 
of the hydride of copper might have absorbed more 


1 This combination is hypophosphorous acid, the action of which 


upon sulphate of copper gives rise to the formation of copper hy- 
dride. 


10 


210 THE ATOMIC THEORY. 


heat: the thermal result is therefore unquestionably 
negative. 

As regards the reciprocal reductions of peroxides, 
Berthelot again observes that hydrogen peroxide, ozone, 
and probably oxide of silver, as well as the metallic 
acids mentioned above, are formed with absorption of 
heat. The fact is here unimportant and the argument 
no longer to the point. For though their instability 
would be rendered intelligible from the admission that 
these metallic acids and peroxide of hydrogen contain 
more heat than the lower oxides to which they are 
reduced, this fact would not explain their reciprocal 
reduction. 

Free nitrogen has a weak affinity for the greater 
number of the other elements, and can only combine 
indirectly with a great number, often with absorption of 
heat. ‘The reason is very simple: the heat liberated by 
the combination of nitrogen with chlorine is less than 
the heat which is absorbed when the diatomic molecules 
of nitrogen and chlorine are resolved into two atoms. 
If, therefore, heat is disengaged by the decomposition of 
nitrogen chloride, it simply proves that more heat is 
liberated on the reconstitution of these molecules 
containing two similar atoms than is absorbed on the 
separation of the atoms in nitrogen chloride. This all 
argues in favour of the modern idea that the molecules 
of certain simple bodies are formed of several atoms 
which exercise a certain mutual attraction, or expend 
upon each other, entirely or in part, the affinities with 
which they are endowed. 


ATOMICITY. 211 


Nie 


Kekulé has made an important advance in this 
direction. This eminent chemist, from a consideration 
of its simplest combinations, was the first to recognise 
the fact that carbon should be regarded as quadrivalent. 
For, in its saturated compounds, a single atom of carbon 
is united with 4 atoms of hydrogen in marsh gas, with 
4 atoms of chlorine in carbon chloride, with 3 atoms of 
hydrogen and 1 atom of chlorine in methyl chloride, and 
with 3 atoms of chlorine and 1 atom of hydrogen in 
chloroform, Again, it is united with 2 atoms of oxygen, 
which are equal to 4 of hydrogen, in carbonic acid gas, 
and in carbon disulphide with 2 atoms of sulphur, 
which are equal to 4 of hydrogen. This is sufficient, for 
though the list of compounds in question is far from 
being complete, the demonstration is so well known 
that further remark is unnecessary. Carbon is therefore 
a quadrivalent, or, in the language of that time, a te- 
tratomic element, which means that its capacity of 
combination with hydrogen is four, while that of nitro- 
gen is three, oxygen two, and chlorine one. The 
following table will show the increasing capacity of com- 
bination of these four elements :— 


Cl’H. hydrochloric acid, 
O”H, water, 

N’’H, ammonia, 

C'vH, marsh gas. 


Moreover, their capacity of combination is equal to 
their substituting value, for, if 1 atom of carbon in com- 


212 THE ATOMIC THEORY. 


bining with hydrogen is saturated with 4 atoms of this 
gas, it will be also able to replace 4 atoms of this gas. 
Thus guanidine, for example, may be regarded as derived 
from 3 molecules of ammonia by the substitution of 1 
atom of carbon for 4 atoms of hydrogen. 


Civ 
N; Hy N; ra 


8 molecules of ammonia. Guanidine. 


This is also the case with the atoms of nitrogen, 
oxygen, and chlorine, which can respectively replace 3 
atoms, 2 atoms, or 1 atom of hydrogen, as in the follow- 
ing compounds :— 


(C,H;)H,NCl (C,H,)N’'NCL1 
Aniline chlorhydrate. Diazobenzene chloride. 
C,H,.0H C,H,0”.0H 
Alcohol. Acetic acid. 
C,H,0.0H C,H,Cl’0.0H 
Acetic acid. Monochloracetic acid. 


Thus the capacity of combination of elements deter- 
mines their substituting value. These two ideas are 
correlative, and are expressed by the term ‘ atomicity.’ 

Atomicity is therefore identical with the valency 
of atoms, and it seems necessary to introduce this term 
into scientific language, for it is clear, and it cannot be 
replaced by that of equivalence, because this value or 
valency is different for different atoms. There are uni-. 
valent, bivalent, trivalent, and quadrivalent atoms, The 
elements are also termed monatomic, diatomic, triatomic, 
and tetratomic, though there is one objection to this 
nomenclature, for the same terms are used with a differ- 
ent meaning to designate the gases or vapours of simple 


‘> 


ATOMICITY. 213 


bodies the molecules of which are formed. of 1, 2, or 4 
atoms. This confusion should be avoided. 

In the series of hydrogen compounds enumerated 
above, the valency of the atoms is indicated by the num- 
ber of hydrogen atoms with which they are severally 
united. The atoms of chlorine are so constituted that 
they can only fix one atom of hydrogen, while the 
oxygen atoms can fix two, the nitrogen atoms three, and 
the carbon atoms four, to form saturated hydrogen com- 
pounds. The capacity of saturation of the carbon atoms 
is therefore four times greater than that of chlorine for 
the same element, the unit of saturation being repre- 
sented by 1 atom of hydrogen. And if 1 atom of carbon 
were united with only 3, or 2 atoms, of hydrogen, one 
unit of saturation would be wanting in the first case and 
two in the second. 

But this is not all. Kekulé has gone further, and 
has shown that the carbon atoms can unite with each 
other, and thus satisfy some of the affinities which are 
inherent to them. ‘This fact is so important that we 
think right to reproduce here the proof of the eminent 
chemist. It is founded upon the fact that in saturated 
hydrocarbons the number of hydrogen atoms never 
exceeds the limit indicated by the formula C,H 
The following are examples :-— 


2n+2° 


Hydrocarbons C,Han+. 


Methane CH, 
Ethane C,H, 
Propane C,H$ 
Butane C,H, 
Pentane C,H, 
Hexane CH, 
Heptane Gis 


tane Cy. 


214 THE ATOMIC THEORY. 


A single atom of carbon can unite with 4 atoms of 
hydrogen, but 2 atoms of carbon can only unite with 6 
instead of with 8, because in the latter case they would 
both be saturated with hydrogen and separated from 
each other, forming 2 molecules of marsh gas. 


C,H, = CH, + CH,. 


In ethane, on the contrary, the 2 atoms of carbon are 
only united with 6 atoms of hydrogen because they have 
mutually exchanged one unit of saturation. ‘This re- 
quires explanation. 

If we take two molecules of marsh gas, CH,+CH,, 
and subtract from each of them an atom of hydrogen, 
we shall obtain two residues CH,, in which the carbon 
atom would no longer be saturated. In losing H it has 
recovered a power of combination which renders it 
capable of again uniting with an atom of hydrogen, or 
of replacing an atom of hydrogen where one is wanting. 
Now the affinity of the carbon atoms for each other leads 
them to interchange this force. We find them riveted 
together by the exchange of one unit of saturation, each 
accompanied by 3 atoms of hydrogen. Such is the 
meaning of the formula 


in which this interchange of units of saturation is indi- 
cated by the strokes which separate the letters.} 


? This notation, now in general use, was employed for the first 
time in the lectures which I gave at the Collége de France during 


ATOMICITY. 215 


This reasoning also shows that 3 atoms of carbon 
cannot combine with more than 8 atoms of hydrogen 
to form a saturated compound. In fact, if we take a 
molecule of ethane, C,H,, which is saturated, and a mole- 
cule of marsh gas, we must deprive each of them of an 
atom of hydrogen before the carbon of the one can 
combine with the carbon of the other. When this sub- 
traction is accomplished there will only remain 8 atoms 
of hydrogen, and one of the carbon atoms of ethane, 
thus impoverished, will be able to unite with the carbon 
atom of methane, which has also been deprived of an 
atom of hydrogen. 

The three carbon atoms of the new hydrocarbon, 
propane, will thus form a chain firmly riveted by the 
very affinities which would have separated them from 
each other. The following formule show the generation 
and the atomic grouping of propane :— 


H He Hi DEL 

| Pah Nagel il 
H-C-H + H-C-C-H—H,—H-C-C-C-H 

| bes ph sl 

Hs HH ee Hey, 
Methane. Ethane. Propane, 


Before proceeding we must warn our readers against 
an error. Expressions of the kind of which we have 
just given an example are not intended to describe the 
position occupied by each atom in space. They indicate 
the relations which exist between the atoms. The pre- 


the summer of 1863, and were published first in Dr. Quesneville’s 
Moniteur scientifique, and afterwards under the title of Legons de 
Philosophie chimique (Hachette, 1864). This subject is discussed 
in pp. 140, 143, 145, 158 and 182 of this treatise. 


216 THE ATOMIC THEORY. 


ceding formula shows the manner in which the hydrogen 
atoms are divided between the three atoms of carbon, 
which are bound together by the interchange of units of 
saturation, thus forming, as it were, the nucleus or 
skeleton of the combination. The links of union in- 
serted between the atoms do no more than mark their 
degree of saturation. They indicate the number and 
the interchange of the units of saturation, and that is 
all. Each atom of the quadrivalent carbon is sur- 
rounded by four strokes, while atoms of the univalent 
hydrogen have only one. 

The line of argument which we have just been 
following applies also to saturated hydrocarbons con- 
taining a larger number of carbon atoms. Carbon 
atoms to the number of 4, 5, or 6 would interchange a 
part of the capacity of saturation which is inherent in 
them. It is clear that the carbon nuclei thus formed 
will only leave 10, 12, or 14 places vacant for as many 
atoms of hydrogen. ‘Thus, to take a final example, 6 
units of saturation are required by 4 atoms of carbon 
to form a firmly riveted chain, and of the 16 units of 
saturation which were contained in the 4 atoms of 
carbon there remain, therefore, only 10 capable of 
fixing atoms of hydrogen. 

The above discussion will show the meaning and 
importance of Kekulé’s great conception. This idea 
explains three facts, which have no apparent connection. 

1st. The fact that no saturated hydrocarbon can 
contain a greater number of carbon atoms than that 
indicated of the formula C,H,,,,.. 

2nd. The fact upon which Laurent and Gerhardt 


ATOMICITY. Sin 


had formerly laid so much stress—namely, that the 
number of hydrogen atoms contained in the hydro- 
carbons is always even. 

3rd. The great stability of these hydrocarbons, 
which is due not only to the great affinity of hydrogen 
for carbon, but also of carbon for carbon. 

These facts, which were revealed by observation, only 
presented an empirical character. They are now ex- 
plained by, and subordinated to, a principle from which 
they flow as natural consequences. The affinity of 
carbon for carbon is the cause of the infinite variety and 
immense number of carbon compounds : it is the essence 
of organic chemistry. No other element possesses in 
the same degree this ruling property of the element 
carbon, the faculty which its atoms possess of combining, 
of becoming riveted together, so as to form that frame- 
work, so variable in form, dimensions, and solidity, which 
acts, so to speak, as a support to the other elements, or 
rather to the atoms of the other elements. The latter 
are not, however, wanting in this property of uniting 
together, to which part of our subject we must now turn 
our attention. 


IV. 


We have described above the theory of diatomic gases 
and vapours. The molecules of hydrogen are formed of 
two atoms which, being univalent and combined with 
each other, have exhausted, by this act of union, all 
the capacity of combination which they possess. The 


218 THE ATOMIC THEORY. 


molecule of hydrogen cannot, therefore, serve as a point 
of attachment to another atom; it represents a saturated 
compound which can only be modified by substitution. 
This is also the case with a molecule of chlorine, and 
when these two molecules are brought into contact with 
each other they are reciprocally decomposed, and hydro- 
chloric acid is formed, as we have already seen, by the 
interchange of the hydrogen and chlorine atoms of these 
diatomic gases. 

The molecule of oxygen, again, is formed of two 
atoms joined together, and as they each possess a ca- 
pacity of saturation which is represented by two units, 
the union of the two atoms may be represented as 
cemented by the interchange of these two units of 
saturation or atomicities. Following the notation indi- 
cated above, this double exchange may be represented 
by two strokes. The molecules of oxygen may be 
written, therefore, O—O=2 volumes. But we may also 
suppose these two atoms of oxygen to be simply united 
by a single unit of saturation: two out of these units 
are therefore left unsaturated, and itis clear that in this 
case a molecule of oxygen may serve as a point of 
attachment to other atoms, which may be fixed by each 
of the two atoms of oxygen. If O=O represents a 
saturated couple, the symbol —O—O— will represent 
a couple which is unsaturated and capable of attaching, 
for example, two atoms of hydrogen. This conception 
explains the constitution of hydrogen peroxide, 
H—O—O—H. | 

Certain peroxides have clearly the same constitution 
as hydrogen peroxide. This is the case with the per- 


ATOMICITY. 219 


oxides of barium and strontium, which may be repre- 
sented by the formule 


The considerations which we have just applied to 
bivalent oxygen apply equally to trivalent nitrogen. 
In free nitrogen we may consider that the two atoms of 
the molecule exchange the units of saturation which 
they possess, thus forming a solid chain which few 
elements have the power of disturbing or interrupting 
directly. It is well known that free nitrogen unites 
directly with a very few bodies. 

This pair of nitrogen atoms N=N represents, from 
a thermal point of view, a more stable system (as having 
given rise to a greater liberation of heat) than a com- 
pound formed by an atom of nitrogen and, for example, 
three atoms of chlorine. But these two nitrogen atoms 
N=N which exchange 3 units of saturation, may only 
exchange 2 or | when, as in the preceding case, it acts 
as a point of attachment to other elements in complex 
combinations. The following are examples taken from 
those very remarkable organic combinations known as 
azo- and diazo-compounds :— 

N  O,H,—N  G,H,-N  C,H,—N, C,H,—NH 

O 


N ewe ON. -Ne On Nn. . ~C.H “NA 
Free nitro- Diazobenzene Azobenzene. Azoxybenzene. Hydrazobenzene, 
gen. chloride, 


We here see at once how the unsaturated pair of 
the two nitrogen atoms may serve as a support to other 
atoms, or as a point of attachment to their affinities, if 
we may muke use of this figurative expression. Wealso 


220 THE ATOMIC THEORY. 


see that it is not only elements such as chlorine, hydro- 
gen, and oxygen which are capable of attaching them- 
selves to the atoms of nitrogen (or others) which are 
unsaturated in their affinity, or which have not exhausted 
their capacity of combination; groups, such as phenyl, 
C,H;, given in the preceding formule, share this pro- 
perty with the elements. Phenyl can play the part 
and take the place of a certain atom of hydrogen, be- 
cause it wants but one atom of hydrogen to become 
benzene. We shall return to this point presently. 

We have so far traced the origin, development, and 
consequences of this modern idea—namely, that the 
atoms of simple bodies can expend upon themselves a 
part or the whole of the capacity of combination which 
they possess. We must now enquire into the meaning 
of this term. We have observed this quality highly 
developed in the atoms of carbon; we have met with it 
again in hydrogen atoms, in oxygen and nitrogen atoms 
—that is to say, inthe ordinary elements of organic com- 
pounds. We must now proceed to show that other 
simple bodies, such as silicon and the metals, also possess 
this property. 

Silicon and titanium may be classed among the 
quadrivalent elements analogous to carbon. We are, 
in fact, acquainted with the tetrachlorides, SiCl, and 
TiCl, Friedel has succeeded in preparing a sesqui- 
chloride and sesquiiodide of silicon. The latter, the 
analogue of sesquichloride of carbon, C,Cl,, has the 
same constitution as ethane (p. 214). The two atoms 
of carbon being united together by the exchange of 
one unit of saturation, there only remain six which 


ATOMICITY. 221 


are, so to speak, free to take up six atoms of chlorine. | 
In the sesquiiodide and sesquichloride of silicon the 

six atoms of iodine and chlorine play the same part, and 

the two atoms of silicon are united together, exchanging 

the fourth unit of saturation, or valency, which each of 
them possesses :— 


Cl Cl Cl cl Cl Cl 
(4 | Bret eal 
cl—C—C—Cl cl-Si—Si—Cl_- Cl—Ti—Ti—Cl 


aA Le! a8) 
Cl. Cl Cl Cl Cl Cl 


Sesquichloride of carbon. Sesquichloride of Sesquichloride of 
silicon. titanium. 


The sesquichloride of titanium shows an analogous 
composition. It must be remarked that the formule 
in question cannot be halved. The vapour density of 
all these bodies has been taken, and their molecular con- 
densation must be expressed by the preceding formule. 

The chlorides of iron and aluminium are analogous to 
the preceding chlorides. The result of the classical 
researches of H. Sainte-Claire Deville and Troost upon 
the vapour density of these chlorides has been to attri- 
bute to them the formule Fe,Cl, and Al,Cl,; and we 
are forced to admit that the two atoms of iron and 
aluminium are united together in the same manner as 
the atoms of carbon, silicon, and titanium in the corre- 
sponding chlorides. 

The couples Fe—Fe and Al—A\ are, then, sexvalent. 
This ingenious idea is due to Friedel. Considering iron 
as quadrivalent in pyrites, FeS,,' the eminent chemist 


} The ferric tetrachloride corresponding to pyrites does not exist. 
The interpretation of this want lies in the fact that in the action 


222 THE ATOMIC THEORY. 


regards the ferric compounds as containing two atoms of 
tetratomic iron united by the interchange of two units of 
saturation. In the couple (Fe—Fe), ferricum, there 
remain, therefore, only six free or disposable units of 
saturation. The violet chromic chloride, and perhaps 
the compounds which are called sesquichloride of osmium 
and ruthenium, have the same molecular constitution 
as the preceding chlorides. 
(Aly—Alry Cl,  [Felv—Fel”]"Cl,  [Cr-—Cr'v] “C1, 


Chloride of Hexachloride of Hexachloride of 
aluminium. iron. chromium, 


[ Ost-—Osi" }"iC], [Ruly—Ru!¥}"!Cl, 
Hexachloride of Hexachloride of 
osmium. ruthenium. 


The corresponding oxides are— 


r 


(Al,)"05 (Fe,)"'0, (Cr,)"05 (Os,)"'0, (Ru,)0s. 


These trioxides must not be confounded with the sesqui- 
oxides properly so called, which contain trivalent 
elements, such as arsenic, antimony, bismuth, and gold. 
These sesquioxides correspond to trichlorides, and the two 
atoms of metal which they contain are united, not 
directly with each other, but through an intermediary 
atom of oxygen. 


of chlorine upon the protochloride a tetrachloride is not formed, be- 
cause the affinity of iron for iron is greater than that of four atoms 
of chlorine for iron. 


FeCl, + FeCl, =(Fe--Fe)"Cl, + C1CL 


We must add that important researches made by Scheurer-Kestner 
upon the ferric salt, have confirmed the existence of sexvalent iron, 
Fe.,. 


ATOMICITY. 223 


As"Cl, Sb’Cl, B’Cl, Aw’’Cl, 
Cnloride of arsenic. Chloride of anti- Chloride of Chloride of gold. 
mony. bismuth. 
ZsaAs"0 Y Sb’”O ZEo sAw"d 
\As””0 \$b””0 \Bi”O \Au”0O 
Arsenious anhydride. Sesquioxide of Sesquioxide of Sesquioxide of 
antimony. bismuth. gold. 


Iridium and rhodium also form well-characterised 
trichlorides and sesquioxides, which seem to belong to 
the preceding series ; but they also form dichlorides, or 
rather tetrachlorides, in which we may admit the ex- 
istence of couples (Ir—Ir) and (Rh—Rh) formed by the 
union of two atoms of iridium or two atoms of rhodium, 
which, having exchanged one unit of saturation, now 
possess only four atomicities. 

(Ir’”—Ir'”)"Cl, (Rh”’—Rh’”)"Cl, 
Dichloride of iridium. Dichloride of rhodium. 

Asa final example of these unions which the atoms of 
the same element may form, by the partial exchange of 
their atomicities or units of saturation, we may mention 
the cuprous and mercurous compounds, of which the first 
contain two atoms of copper, the second two atoms of 
mercury, united together. 


(Cu —Cu" CL ( Hge"”—He" yCl, 
Cuprous chloride. Mercurous chloride. 
(Cu"”—Cu" yo (Hg”—He”)’0 
Cuprous oxide. Mercurous oxide. 


The formula which is here attributed to mercurous 
chloride has been amply justified (p. 115), whence it 
seems allowable to attribute an analogous composition 
to cuprous chloride, though here there is some un- 
certainty. 


224 THE ATOMIC THEORY. 


CHAPTER II. 


if 


Affinity and Atomicity, two Distinct Properties of 
Atoms. 


WE have in the preceding pages defined atomicity by 
regarding it as the saturating capacity of atoms, or as 
their valency in combinations. It is, then, a property 
inherent in the nature of atoms. We must proceed to 
show how it differs from affinity. 

Affinity is the force of combination, chemical 
energy. It determines the intensity and the direction 
of chemical reactions, and is estimated by the thermal 
effects which these reactions produce. It varies essen- 
tially with different atoms. In combining with atoms 
of hydrogen, atoms of chlorine, iodine, and bromine 
liberate very different quantities of heat; their affinity 
for hydrogen is very different, and is proportional to the 
quantities of heat liberated. But if we consider the 
combinations of the same elements with oxygen we shall 
find the order of affinities reversed. Chlorine is the 
element which possesses the weakest affinity for this 


AFFINITY AND ATOMICITY. 225 


body. The compounds of chlorine and oxygen are very 
unstable ; some decompose with explosion—that is to 
say, are formed with absorption of heat. The affinity or 
chemical energy of a given body must therefore be con- 
sidered as a relative property. It depends upon the 
nature of the element with which the one in question 
combines. 

It depends also upon the conditions under which the 
bodies are placed. Berthollet long ago showed the 
influence which is exercised upon affinity by physical 
conditions, such as the degree of cohesion and the in- 
solubility of bodies. This fact is too well known to 
require further remark (see p. 4); but we must 
remember how physical agents, such as heat, light, or 
electricity, can augment or diminish chemical energy, 
stimulate or retard the exercise of affinity. If mercury 
is heated to a certain temperature its atoms are in a 
condition capable of attracting atoms of oxygen. Ifthe 
heat is inereased the atoms of mercury and oxygen will 
be separated again. The affinity of mercury for oxygen 
is therefore subordinate to the temperature. Itisa rela- 
tive and not an absolute property, like the atomic weight. 
In the same manner a stream of electric sparks or the 
silent electric discharge can determine combinations 
between atoms which would have no action upon each 
other under ordinary conditions. Inversely, the same 
influences can produce decomposition, as is the case with 
the battery current. Here, again, the conditions in 
which the atoms are placed exercise a visible influence 
upon their affinities. 

Atomicity is the capacity of saturation, or the value 


226 THE ATOMIC THEORY. 


of substitution possessed by atoms, and this valency 
is an essentially different thing from the force of com- 
bination or the energy which resides in them. - It 
governs the form of combinations, which varies with 
each atom. Thus the hydrogen combinations of chlorine, 
oxygen, nitrogen, and carbon have a different-form (p.° 
211), and the atoms of ‘carbon are so constituted that 
they can attract four atoms of hydrogen, whilst nitrogen 
can onlyattract three, &c. Weshould, moreover, observe 
that the force with which the hydrogen atoms are 
attracted by these different simple bodies is independ- 
ent of the number of atoms fixed in each case. Thus 
we know that while hydrogen is united to chlorine with 
extreme energy, oxygen combines with less force, carbon 
with difficulty and only when excited by most powerful 
influences, and nitrogen not at all directly. 

These two notions, affinity and atomicity, which 
form the very foundation of the science, are therefore 
essentially different. 


If. 
Atomicity a Relative Property of Atoms. 


Let us pursue this parallel. Is the atomicity or capa- 
city of saturation of every kind of atom immutably fixed, 
whatever the combinations may be into which they enter? 
By no means. The action of atoms must be regarded as 
reciprocal, so that in a compound formed of two hetero- 
geneous atoms the properties of the one are influenced 


ATOMICITY A RELATIVE PROPERTY. 227 


by those of the other, the two atoms adapting themselves, 
as it were, to each other. Atomicity is therefore a rela- 
tive proeprty, like affinity. This is easily proved to be 
the case. Nitrogen, phosphorus, arsenic, and antimony 
only combine with three atoms of hydrogen ; the three 
latter elements also combine with three atoms of chlorine; 
but while phosphorus and antimony can unite with five 
atoms of chlorine to form the pentachlorides, arsenic can 
only unite with three atoms of this element. Here, 
therefore, we have essential differences in the saturating 
capacities of simple bodies for hydrogen and chlorine. 
The hydrogen compounds exhibit a particular form and 
belong to a certain type, the same for all; the chlorine 
compounds do not exactly correspond, phosphorus and 
antimony, but notarsenic, forming with chlorine chlorides 
which belong to a particular type. 

Let us now consider some other compounds formed 
by the same group of bodies. We do not know of one 
of them forming a hydrogen compound or an ethyl or 
methyl compound belonging to the type RX,; but 
nitrogen, which can fix neither five atoms of hydrogen 
nor five ethyl groups, is united in sal ammoniac to four 
atoms of hydrogen and one of chlorine, and in tetrethyl- 
ammonium iodide to four ethyl groups and to one atom 
of iodine. 


NH, + HCl = NH,CI; 
NEt, + EtCl = NEt,Cl. 


Phosphorus, arsenic, and antimony also form the com- 
pounds— 


228 THE ATOMIC THEORY. 


PEt, I AsMe,(Cl 

SbH,I AsMe,Cl, 
AsMe,Cl, 
AsMeCl, 


which belong to the type RX,. 

The methyl compounds of arsenic are worthy of 
attention from our present point of view. Arsenic can 
neither combine with five atoms of chlorine nor with 
five methyl groups; but well-defined compounds are 
known containing for one atom of arsenic four methyl 
groups and one atom of chlorine, or four atoms of chlorine 
and one methyl group, whence it appears that the com- 
bining capacity of arsenic varies, and is in a manner 
increased when chlorine and methyl are both present to 
enter into combination with arsenic. 

The oxygen compounds of the bodies in question 
belong generally to the types RX, and RX,. But here 
again we meet with peculiarities worthy of notice. 
Nitrogen is bivalent in nitrogen dioxide, NO,! which 
compound is not saturated. It is quadrivalent in NO, ; 
but this latter, again, tends to unite with itself at a low 
temperature, thus forming the body O,N’—N’‘0O,. 

Arsenic forms with sulphur a compound AsS, or 
As,S,=S,As’—As‘S,, which has no analogue in the 
oxygen series. 

We may conclude, therefore, that, as far as nitrogen 
and its congeners are concerned, there is no absolute 
rule for the saturating capacity of atoms, since we find 
that the latter varies with the nature of the elements or 


‘Or —N’’0. 


ATOMICITY A RELATIVE PROPERTY. 229 


groups which are united with the simple bodies in 
question. 

Let us now consider the chlorine family. This body 
and its congeners behave towards hydrogen, ethyl, and 
the metals as univalent elements. 

C1H hydrochloric acid. 
ClEt’ chloride of ethyl. 
CIK chloride of potassium. 


Cl,Pb” chloride of lead. 
Cl,Sb” chloride of antimony, &c. 


This is not the case with the different oxygen 
compounds of chlorine and its congeners, in which the 
saturating capacity of these elements for oxygen is 
exhausted by degrees. Thus in hypochlorous acid, 
Cl(OH)’, chlorine is univalent ; it is quinquivalent in 
chlorie acid, ClO,(OH), and septivalent in perchloric 
acid,' ClO,(OH). With perchloric acid we may com- 


1 Some time ago I expressed the idea that in certain oxygen 
compounds rich in oxygen the atoms of this body might be united 
in such a manner as to form a chain. Thus I represented the 
constitution of chloric acid and perchloric acid by the formule— 


Cl’O—O—O—(0OH) Cl’—O0—O—O—(OH)Y' 


Chioric acid. Perchloric acid. 


This hypothesis afterwards received support from ideas upon the 
constitution of the quinones. We know that Graebe and Lieber- 
mann regarded quinone as a benzene derivative, in which the diato- 
mic group (O—Q)” was substituted for two atoms of hydrogen 
in benzene, 


CH 0,H,(0—0y" 


Benzene. Quinone. 


This idea had to be abandoned, and I must give up my old hypo. 
thesis upon the constitution of the acids of chlorine, sulphur, &c. 


230 THE ATOMIC THEORY. 


pare permanganic acid, MnO,(OH), where manganese 
is septivalent ; it is bivalent in the dichloride Mn(Cl,, 
probably quadrivalent in the peroxide MnO,, &c. 

Todine, which belongs to the family in question, pre- 
sents a noticeable peculiarity ; it forms with chlorine a 
protochloride, ClI, in which it appears to play the part 
of an univalent element, as in iodide of potassium, a 
saturated compound. In the protochloride the iodine 
is not saturated, for it can fix two more atoms of chlorine 
to form a trichloride. And this trichloride of iodine 
is unquestionably an atomic compound, for the three 
atoms of chlorine may be replaced by three acetyl groups 
(Schutzenberger). 

Thus we have the following compounds :— 


VCl I"Cl, I’"(C,H,0,)s 
Iodine protochloride. Iodine tri- Iodine triacetate. 
chloride. ‘ 


In iodic acid, 10,(0H)’, iodine is quinquivalent ; it 


The existence of a chain of oxygen atoms in the higher acids of 
chlorine seemed scarcely to accord with the stability of these 
acids, increasing as it does in proportion to the number of oxygen 
atoms. I therefore incline to the idea that chlorine is heptatomic 
or septivalent in perchloric acid, and that sulphur is sexvalent in 
sulphuric acid. Given the fact of multiple proportions, if we 
admit that atomicity varies by degrees, there is no reason why we 
should not admit that a given element may manifest towards 
oxygen a capacity of combination seven times greater than to- 
' wards hydrogen. We must, however, add that in certain per- 
oxides analogous to hydrogen peroxide we must admit a similar 
existence of two atoms of oxygen united to each other. 


on Ba 
Le io 
O—O a 


Hydrogen peroxide. Barium peroxide. 


ATOMICITY A RELATIVE PROPERTY. 231 


is septivalent in periodic acid, 10,(OHY. This latter 
acid forms very remarkable hydrates, and also corre- 
sponding definite salts.! These hydrates are— 


10,H+H,0 =I*#0,(0H),; 
10,H + 2H,0 =I""0(OH),. 


The polyatomic character of iodine is much more 
striking than that of chlorine, and it is worthy of re- 
mark that iodine can fix in the different hydrates of 
periodic acid one, three, or five hydroxyl groups, form- 
ing relatively stable compounds. 

Let us now pass to another family, that of oxygen, 
which is a bivalent element. Sulphur, selenium, and 
tellurium are also bivalent in their hydrogen compounds. 
They are quadrivalent in the anhydrides SO,, SeO,, TeO,, 
and in the chlorides SeCl,, TeCl, ; sexvalent in the anhy- 
drides SO,, SeO,, TeO,, and in sulphuric acid, SO,(OH),, 
selenic acid, SeO,(OH),, and telluric acid, TeO,(OH),. 
Oxygen, which belongs to the same family, is one of the 
most strongly characterised bivalent elements. Can it, 
like its congeners, in some cases act as a quadrivalent 
element? This is not impossible, and the supposition 
receives support from an important discovery made by 
Friedel. Methyl oxide, (CH,),O, will unite with hydro- 
chloric acid, HCl, although both bodies may be regarded 
as saturated, and the combination is so stable that it is 
not completely dissociated at its boiling point. If the 
molecule (CH,),O.HCI can exist in the state of vapour, 


OH 


1 Ordinary sodium periodate is 10, + 
\X(ONa) 
g 


H,0. 


232 THE ATOMIC THEORY. 


the hypothesis of quadrivalent oxygen would account 
for this fact: chlorine and hydrogen can be attracted at 
the same time as the two methyl groups. (Friedel.) 

The development of supplementary atomicities in 
oxygen would account, as Friedel has recently remarked, 
for the formation of certain compounds called molecular, 
notably for the fixation of water of crystallisation by a 
great number of anhydrous molecules. But this is con- 
nected with a general question which will be treated 
presently. 

We may here draw attention to a remark which is 
not devoid of interest. Oxygen is bivalent; sulphur, 
selenium, and tellurium exhibit, in a great number of 
cases, higher atomicities. Again, in another family 
chlorine is univalent, at least as far as its combinations 
with hydrogen and the metals are concerned ; iodine, 
however, manifests higher atomicities. Does it not 
seem as if this tendency to develope atomicities of a 
higher order might bear some relation to the increase 
of the atomic weight ? for in the same family the heaviest 
elements seem more apt than the others to form com- 
binations of a higher order—that is to say, to display 
higher atomicities. 

Chromium possesses some analogy with sulphur, so 
much so that Mendelejeff places it, with molybdenum 
and tungsten, in the oxygen andsulphur group. In this 
metal the sexvalent character is even more pronounced 
than in sulphur; it becomes more so in molybdenum 
and tungsten, the atomic weights of which are higher, 
and which we know form hexachlorides. But the 
chlorine compounds of tungsten offer a striking example 


ATOMICITY A RELATIVE PROPERTY. 233 


of the variation of atomicity in the same element. Not 
to mention the dichloride of tungsten, three other well- 
defined chlorides are known, namely— 

WCl,, 

WCl,, 

WCl,, 
in which tungsten evidently possesses a valency or com- 
bining value which differs as the numbers 4, 5, 6. 

Let us consider some other metals from the present 
point of view—that is to say, of variable atomicity and 
of its tendency to augment in value with the increase of 
atomic weight. 

Iron is bivalent in the dichloride, quadrivalent 
in the disulphide FeS, and in the _hexachloride 
(Fe—Fe)"Cl, (Friedel); but the dioxide corresponding 
to the disulphide and tetrachloride is unknown—a 
fresh proof that atomicity is dependent upon the nature 
of the two combining elements. 

Ruthenium! forms a well-defined tetrachloride, but 
the hexachloride of ruthenium is unknown. Such a 
combination is formed, however, by osmium, the ana- 
logue of ruthenium, the atomic weight of which is 
higher.. We may add that in perruthenic acid and in 
osmic acid, which is so stable, ruthenium and osmium 
act as octovalent elements. 

In the same manner we may compare rhodium to 
iridium, palladium to platinum ; then again the alkaline 
metals to silver, to gold,and to thallium. We will con- 


1 Tron, ruthenium, and osmium form a series in Mendelejeff's table 
(pp. 159, 160). 
jh! 


234 THE ATOMIC THEORY. 


fine ourselves to the latter comparison.! The alkaline 
metals and silver are univalent. Gold, the atomic 
weight of which is higher than that of silver, forms 
not only a protochloride, but also a well-characterised 
trichloride. It is the same in the case of thallium 
compared with cesium and rubidium. With all these 
metals atomicities of a higher order are developed as the 
atomic weight increases. 

Let us turn to carbon as a last example in this 
long discussion. 

Following the example of Kekulé, we have con- 
sidered carbon as quadrivalent in the saturated com- 
pounds which it forms with oxygen, sulphur, hydrogen, 
and chlorine. But there are other combinations of 
carbon, in which this element is not saturated. Carbon 
monoxide, CO=2 volumes, furnishes an example. In 
this body carbon has not exhausted its capacity of com- 
bination for oxygen, since it can fix another atom to. 
form carbon dioxide, CO,. 

Nor is its affinity or its combining energy exhausted 
in carbon monoxide, since this gas evolves heat when 
combining with oxygen; and yet carbon monoxide 


1 The group of alkaline metals properly so called comprises the 
following metals :— 


Li, Na, K, Rb, Cs, 


to which a sub-group may be added, comprising silver, copper, gold, 
and thallium. Copper seems misplaced here, and yet several 
reascns may be brought forward in favour of the connection of 
this metal with silver, amongst others the isomorphism of Cu,S 
and Ag,S (p. 141). As to thallium, we are evidently authorised in 
connecting it with the alkaline metals, although in Mendelejeff’s 
table it is placed in another series. 


ATOMICITY A RELATIVE PROPERTY. 236 


represents a stable molecule, a definite though un- 
saturated combination. It still retains an affinity for 
oxygen as an active force, without manifesting it as 
long as it remains carbon monoxide. This molecule 
differs both in form and type from that of carbon di- 
oxide, and if we consider the units of saturation which 
are exchanged in the two combinations, we shall find 
that there are two in carbon monoxide and four in the 
dioxide. It follows, therefore, that in carbon monoxide 
the carbon atom plays the part of a bivalent element, 
while it is quadrivalent in carbon dioxide. This, how- 
ever, is in reality but a figure of speech, for we may 
add that if it does not manifest to the full extent the 
capacity which it possesses for oxygen, it is not the less 
true that it possesses it, since it will manifest it as soon 
as occasion offers. Carbon monoxide contains an atom 
of carbon which is still in possession of two units of 
saturation, as may be.expressed by the following for- 
mula: C—O”, carbon dioxide being 0’ =C'*—=O”. 
It would be waste of time to propose and discuss the 
question of variable atomicity, if it could be reduced to 
these terms. But this is not the case. Cooper was the 
first to observe that carbon occurs in a great number of 
compounds in the condition of the carbon in carbon 
monoxide. It is important to examine into and esta- 
blish this statement, for the highest aim of chemistry is 
to discover the constitution of bodies, to determine the 
grouping and mutual relations of atoms, to define, con- 
- sequently, the part which each plays with regard to its 
neighbours ; and if, amongst these atoms, there are 
some which have not exhausted their capacity of com- 


236 THE ATOMIC THEORY. 


bination, they must be distinguished from the others 
and marked with a characteristic sign. This would be 
of great assistance in understanding constitutional 
formule and in interpreting chemical reactions, for it 
must not be forgotten that the properties of bodies are 
dependent upon their constitution. 

Take, for example, the two isomeric bodies methyl 
cyanide and the methylcarbylamine of A. Gautier. 
Their composition is expressed by the formula C,H,N, 
which gives no information as to the causes of their 
isomerism. This is most satisfactorily explained by the 
rational formule proposed by Gautier — 


O44 
N’”’=C'"_CH, wrt 
CH, 


Methyl cyanide. Methylcarbylamine. 


The first represents a compound of cyanogen. The 
trivalent nitrogen exhausts its capacity of combination 
in exchanging three units of saturation with the quad- 
rivalent carbon. The group (CN) is therefore univalent, 
for the carbon is not saturated. It is cyanogen, and 
can fix methyl by its unsaturated carbon. The methyl- 
carbylamine is a base, an ammonia compound containing 
trivalent nitrogen. The latter exchanges one unit of 
saturation with a methyl group, and two units with an 
atom of carbon which here takes the place of two atoms 
of hydrogen. In fact, we might say that methylcarby- 
lamine was derived from methylamine, in which the 
two hydrogen atoms are replaced by an atom of bivalent 
carbon. This is perfectly expressed by the term methyl- 
carbylamine. 


ATOMICITY A RELATIVE PROPERTY. 237 


It is surely scarcely necessary to add that is not 
merely a theoretical view, but that the preceding 
formule interpret reactions, and are to a certain extent 
nothing more than the abridged and commodious repre- 
sentation of those reactions.' 

We say, therefore, that carbon is contained in the 
two isomeric compounds in question under two differ- 
ent forms, quadrivalent in methyl cyanide, bivalent in 
methylearbylamine, saturated in the former and, if you 
will, unsaturated in the second. And it is well to re- 
member this, since the above notation serves to repre- 
sent the constitution of bodies—that is to say, the 
reciprocal relations between atoms—and to interpret the 
accompanying reactions. 

We must add a last example to the preceding, which 
we have chosen from a number of others. 

Urea is an amide—that is to say, a derivative of 
ammonia—and the two atoms of nitrogen which it con- 
tains have the same value and are united to the same 


1 Carbon is united to carbon in methyl cyanide. This body 
yields, by the action of potash, acetic acid, where carbon is united 
to carbon. 


CH, CH, 
s Some ts et § poe malo ta fete onan gs: 
ON CO,H 

Methyl cyanide. Acetic acid. 


The two atoms of carbon are united to nitrogen in methylcarbyla- 
mine, and consequently separated from each other, They are also 
separated by the action of potash, the one remaining united to 
nitrogen in methylamine, the other giving formic acid. 


o” CH 
ng + 2H0 = NC + H.COE 
CH, H 


- 
Methylcarbylamine. Methylamine. Formic acid. 


238 | THE ATOMIC THEORY. 


atom of carbon; separated from each other, they are 
both trivalent. The isomer of urea, isocyanate of am- 
monium, contains nitrogen in two conditions: one atom, 
united to the carbonyl group, is trivalent; the other, 
which with four atoms of hydrogen forms the ammonium 
group, is quinquivalent. The following formule repre- 
sent, therefore, the constitution of these two bodies, 
which can be transformed one into the other :— 


N”’H : Cor 
O= ore 3 wid 
N’'H, \NrH, 
Urea. Isocyanate of ammonium. 


In this case a change in the state of saturation of 
nitrogen accurately determines and explains the trans- 
formation of isocyanate of ammonium into urea, and 
of urea into isocyanate of ammonium. 


KE 


Now, what have we proved by the preceding 
remarks? We have endeavoured to establish that 
atomicity is not more immutable than affinity itself, 
but that it isa relative property of atoms. It varies, in 
fact, withthe same element in the different combinations 
which the element is capable of forming with other ele- 
ments, according to the nature.of the latter, and in the 
combinations which it is capable of forming with the 
same simple body, according to the condition of satura- 
tion of the compound in question. It varies also with 
the temperature, for it is well known that, with regard 


ATOMICITY A RELATIVE PROPERTY. 239 


to certain elements, certain forms of combination can 
only exist within very narrow limits of temperature. 

These variations in the combining capacity of atoms 
are evidently a part of their intimate nature, of their 
form of existence. They probably depend upon the 
different velocities of the atoms. When two hetero- 
geneous atoms come within their reciprocal spheres of 
action, they cannot unite unless their velocities are 
of a special character: there must be an accommoda- 
tion, which is mutual. It determines the form of the 
combination, and also the form and dimensions of the 
new molecule in space. ‘This is why the combining or 
saturating capacity of a given element is only a relative 
property; it cannot be the same towards the atoms of 
all elements, for each of the latter has its own indivi- 
duality, its own velocity, which require a special 
character in that of the atom which enters into 
combination. The fundamental properties of the one, 
its chemical energy and capacity of combination, are 
influenced by the properties of the other in a manner 
which varies with the nature of the latter. 

In the second place, it must be remembered that in 
the multiple compounds which one element forms with 
another, the state of saturation of the former varies. 
We are taught this fact by the law of multiple propor- 
tions. We know that the affinity of one element for 
another is exhausted by degrees, and these degrees 
accurately mark the state of saturation of the former. 
In this respect, then, the theory of atomicity is nothing 
more than the renewed and revived expression of the 
law of multiple proportions, as we remarked thirteen 


240 THE ATOMIC THEORY. 


years ago.! Does this mean that the two conceptions 
are identical, and that the former shows no advance 
upon the latter, and is, in consequence, superfluous ? 
Such an opinion would not be tenable for a moment. 
There is a great difference between the law of multiple 
proportions, which is only the direct expression of an 
experimental fact, and this studied theory, which 
consists in seeking for each simple body the forms of 
combination by which it is characterised, in comparing, 
in this respect, the elements with each other, in attri- 
buting to each of them a capacity of saturation which 
may vary in every compound, but which is perfectly 
definite in a given compound, in discovering the bearing 
of this property upon the constitution of chemical com- 
binations, or how each atom exhausts in uniting with 
other atoms, whether of adifferent or of the same nature, 
the capacity of combination which it possesses, and in 
making use of these data to establish the probable 
relations between atoms in compounds, and, conse- 
quently, to construct the molecular edifice. This latter 
point is so important that we feel forced to return to it. 
But before closing the discussion now occupying our 
attention we must endeavour to explain a delicate 
point. Elements whose degree of saturation does not 
vary—such as hydrogen and, to a certain extent, the 
alkaline metals—are very easily characterised. They 
are univalent. This is not the case with those which 
form multiple compounds. Are phosphorus and nitro- 
gen trivalent elements? They are so in the greater 
number and in the more stable of their compounds. 


1 Legons de Philosophie chimique, p. 221. 


ATOMICITY. 241 


In others they are quinquivalent. This is the case with 
nitrogen in sal ammoniac, where it is united to five 
univalent elements, four of hydrogen and one of chlorine. 
And the very reason why ammonia can unite with hydro- 
chloric acid is because the nitrogen which it contains 
is not saturated to its fullest extent. It is saturated 
as regards hydrogen, but not as regards hydrochloric 
acid. 

The same difficulty arises with respect to phos- 
phorus, arsenic, and antimony (see p. 227). It has 
been supposed at one time that these elements were 
trivalent, at another that they were quinquivalent. 
Setting aside the question as to what they are absolutely 
with regard to themselves, we may say that they act as 
trivalent elements in one order of compounds, and quin- 
quivalent elements in other compounds. This is suffi- 
cient not only to determine the atomic structure of these 
compounds and of those which are derived from them, 
such as the acids of nitrogen, phosphorus, and arsenic,! 


1 The following examples are well calculated to show the prac- 
tical utility of these considerations upon atomicity. The accom- 
panying formulz represent the composition of the two series of 
compounds mentioned in the text :— 


N’’H, Pee. As'"H, 

Ammonia. Phosphine. Arsine. 

BVO AsCl, 

Trichloride of phosphorus. Chloridé of arsenic. 
N”"(OH), P’"(OH), As!"(OH), 
Normal nitrous acid. Phosphorous acid. Normal arsenious acid 
(unknown). 
O 
int 
N \OH ” bb] 


Nitrous acid 
{first nitrous anhydride). 


242 THE ATOMIC THEORY. 


but also to interpret the mode of formation and the re- 
actions of all these bodies, which is the essential point. 


Ni = eft 
aah P""(OH), "6 ean! 
\N = 0 Aga O 
Nitrous anhydride. Phosphorous acid. Arsenious anhydride. 
N’H,Cl PrH,I As’Me,I 
Ammonium chloride. Phosphonium iodide. Tetramethylarsonium iodide. 
. PrCl, As*MeCl, 
Phosphorus pentachloride. Monomethylarsine 
tetrachloride. 
N*(OH), Pv(OH), As*(OH), 
Normal nitric hydrate Normal phosphoric Normal arsenic hydrate 
(unknown). hydrate (unknown). (unknown). 
O=N*(OH), O=P*(OH), O = As*(OH), 
Orthonitric acid (unknown). Orthophosphoric acid Orthoarsenic acid (first 
(first anhydride). anhydride). 
XT wit a on 
O = N*(0,Bi’”) es _P*(OH), Cate 
O=—~P(OH), O=~As(OH), 
Subnitrate of bismuth Pyrophosphoric acid Pyroarsenic acid. 
(orthonitrate) (second anbydride). 
ON ON On 
N*(OH PO - 
Or) Os he o7as (OD) 
Nitric acid. Metaphosphoric acid (third Metarsenic acid. 
anhydride). 
0 JNO, 0 Paes e 0 LAsO; 
\NO, \PO, \AsO, 
Nitric anhydride. Phosphoric anhydride. Arsenic anhydride. 


We now understand the importance of the principles discussed in 
the text (p. 241). Without touching upon the question of the deter- 
‘mination of the absolute saturating capacity of the atoms of nitro- 
gen, phosphorus, and arsenic (and it is perfectly clear that they are 
relative), we simply take note of that which they manifest in a 
series of compounds, and make use of these data to establish rela- 
tions of saturation between the atoms, and, to a certain extent, to 
account for the structure of molecules. It is very simple for the 
hydrogen and chlorine compounds; it becomes more complicated 
for certain oxygen derivatives. But we cannot fail to be struck 
with the light which the notation derived from considerations upon 


ATOMICITY. 243 


It is evident from what has been said that we 
should encounter serious difficulties if we attempted to 
assion to each element a definite capacity of saturation, 
a fixed atomicity. In the case of certain polyatomic 
elements we should be embarrassed in our choice, for 
it is sometimes difficult to mark the limit of saturation. 


atomicity throws not only upon the constitution, but also upon the 
mode of formation and upon the properties of these acids. Let us 
take a single example, the most complicated one. 

On moderately heating ordinary phosphoric or orthophosphoric 
acid, it is converted into pyrophosphoric acid. Now, the analysis 
of pyrophosphoric acid and the pyrophosphates shows that thisacid 
only differs from orthophosphoric acid by half a molecule of water. 
The concurrence of two molecules of acid is therefore necessary 
for the formation of one molecule of water, and the residue of these 
two molecules remain united by an intermediary atom of oxygen, 
which suffices to saturate the phosphorus of the two molecules. 
This is expressed by the following equation :— 


OH OH HO 
O=PZ0H + shins Oe FO" OnP™ On HOSPr=0 


OH HO ee ee 
_ Orthophosphoric Orthophosphoric Pyrophosphoric 
acid. acid. acid. 


The molecule of pyrophosphoric acid is therefore more complicated 
than that of phosphoric acid, and it is clear that it should be tetra- 
basic, as it contains four atoms of basic hydrogen. Thus the con- 
stitution, the mode of generation, and the fundamental properties 
of pyrophosphoric acid are clearly indicated by the formula 


OH HO 
0=-P4OH HO Pr=0. 
ee a aes 


O . 

The formulz of phosphoric, pyrophosphoric, and phosphorous acidsare 
founded upon considerations relative to the atomicity or valency of 
the atoms of phosphorus and oxygen. Now, I ask, could the law 
of multiple proportions, as it was understood some years ago, have 
given any information upon the atomic structure of all these mole- 
cules? Thus we were justified in our assertion that it was necessary 
to renew and revive this law to explain all these characteristics. 


244 THE ATOMIC THEORY. 


Nothing is easier for hydrogen, oxygen, boron, silicon, 
and a great number of metals. Hydrogen, the alkaline 
metals, and siiver may be classed with the univalent 
elements; the alkaline earths—magnesium, zinc, cop- 
per, &c.—are bivalent. This estimation, however, does 
not apply to other elements, nor can they be character- 
ised by their degree of atomicity, as the latter varies ac- 
cording to the degree and the nature of the combina- 
tions considered. 

Those chemists who hold that atomicity is a fixed 
property of atoms, as invariable as their atomic weights, 
are guided by other considerations. They chose certain 
forms of combination, certain types which, more stable 
or more important than others, seem to them charac- 
teristic of a given element, and suitable for fixing its 
atomicity. Thus the type NX, has been taken as charac- 
teristic of bodies belonging to the nitrogen family ; 
nitrogen and its congeners have therefore been regarded 
as trivalent. But here a difficulty arises. We know 
that the simple bodies in question have a great tendency 
to form more complicated compounds belonging to the 
type NX,. What part, then, can they play in the latter 
compounds? They are, they say, trivalent, like the rest. 
In fact, they admit that the compounds NX, are not 
true atomic combinations, in which all the atoms are 
united so as to form a single molecule ; they are divided, 
so to speak, into two groups, forming two distinct 
molecules combined together, NX, =NX,+4+X,. Hence 
we have two kinds of combinations, atomic combina- 
tions, in which the molecule forms two volumes of 
vapour, and molecular combinations, in which one 


ATOMICITY. 245 


molecule is added to another molecule, and which, 
when they assume a gaseous form, occupy four volumes 
of vapour. This is the case with phosphorous pentachlo- 
ride, with phosphonium iodide, with sal ammoniac, «ce. 
This view has already been refuted. The combinations 
in question are true chemical compounds, and are 
merely dissociated and decomposed when heated (p. 111 
et seq.) 

There seems to me a difficulty in admitting that a 
chemical compound properly so called can be formed by 
the juxtaposition pure and simple of two molecules, 
which are attracted as such and preserve a sort of in- 
dividuality after having contracted this union. Why 
_ does ammonia attract hydrochloric acid? Because the 
nitrogen which it contains is not saturated. This 
must be clearly understood. We admit that sal am- 
moniac, NH,Cl, belongs to the type NX,, and hold 

generally and implicitly that the chlorine and the four 
atoms of hydrogen are united individually to the 
quinquivalent nitrogen. But can chlorine give up its 
affinity for hydrogen and unite with nitrogen, which 
only has a slight attraction for it? This is a difficulty 
which was raised some time ago by Chevreul, and which 
appears to be increased by thermal considerations. The 
separation of chlorine and hydrogen should give rise to 
a considerable absorption of heat ; the union of chlorine 
and nitrogen can only produce a feeble evolution of 
heat. The thermal result of the reaction should, there- 
fore, be negative, and the formation of sal ammoniac 
- should give rise to an absorption of heat. The contrary, 
however, takes place. This difficulty disappears if we 


246 THE ATOMIC THEORY. 


admit that in ammonium chloride the affinity of the 
chlorine for hydrogen is satisfied not by its union with 
a certain atom of hydrogen, but by the attraction which 
it exercises upon all the atoms of hydrogen within the 
sphere of which it is now situated. 

Ammonia combines at a low temperature with 
hydrochloric acid because a residue of energy and 
affinity is retained by the nitrogen, and perhaps also by 
the chlorine. This combination creates a new state of 
equilibrium between all the elements, producing a 
radiation, so to speak, of the atomic affinities and attrac- 
tions of the atoms of nitrogen, hydrogen, and chlorine. 
This is the part played by affinity. | 

The atoms of hydrogen and chlorine unite with a 
great disengagement of heat, and seem to have ex- 
hausted their reciprocal affinity, and yet when the 
molecule of hydrochloric acid is placed within the 
sphere of action of the ammonia molecule there follows 
a fresh disengagement of heat. The reason of this 
is the following: the two molecules, free in their mo- 
tions before combination, are not so afterwards; they 
are bound together, and henceforth execute their mole- 
cular and intermolecular motions with a certain in- 
tensity and in a definite manner, as a single system 
having a common centre of gravity. The fact of 
combination, therefore, produces in the end a loss of 
energy, and in this case, as in others, the final effect 
may be a resultant of many concomitant phenomena 
which are superposed—namely, variation of molecular 
energy and variation of atomic energy. This is the 
cause of the disengagement of heat. 


ATOMICITY. 247 


Ammonia can unite with hydrochloric acid because 
the nitrogen atoms are so constituted, or, if you will, 
are animated by such motions, that they can admit 
into their system not only three atoms of hydrogen, 
but a fourth atom of hydrogen and an atom of chlorine, 
and that the motions of these five atoms can har- 
monise with those of nitrogen in a new system having 
a certain form and certain dimension in space. Such 
is atomicity. 

We say, therefore, that hydrochloric acid can unite 
with ammonia for two reasons—firstly, because the 
atoms uniting are in possession of a residue of affinity ; 
secondly, because the atoms of nitrogen can admit 
into their sphere of action a fourth atom of hydrogen and 
an atom of chlorine. 

The difference between the two notions is evident 
from this example. We see also that we refer the 
faculty which ammonia possesses of attracting hydro- 
chloric acid to a peculiar state, to a fundamental pro- 
perty of the atoms of the former. In admitting the 
existence of atoms we employ an hypothesis; our con- 
ception must embrace as much as possible to allow the 
deduction of all facts and to avoid the necessity of 
creating and employing secondary hypotheses. Chemical 
molecules are formed of atoms which attract each other. 
Such is the hypothesis. I know well that the atoms 
are invisible and inappreciable to the senses, and I do 
not believe that the direct proof of their existence and 
mutual attraction can ever be furnished. But this 
atomic attraction is only a form of universal attraction, 
and as an hypothesis equally legitimate. Why should we 


248 THE ATOMIC THEORY. 


graft upon this hypothesis a second, a special attraction, 
which in a completed combination is exercised by one 
molecule upon another? It seems to us more probable 
that these so-called molecular combinations do not 
essentially differ from atomic combinations, and that 
the explanation les in the properties of the atoms 
themselves. 


IN. 


This is a convenient place to introduce some 
developments of the subject of so-called molecular 
combinations. 

When calcium chloride is placed in water, an 
evolution of heat takes place, which indicates a chemical 
action. A combination has taken place, and the mole- 
cule of calcium chloride, which appears to us completely 
saturated, has nevertheless attracted one or several mole- 
cules of water. In my opinion this chemical action 
was not determined by the molecules of the calcium 
chloride and the water, but by the atoms contained in 
these molecules which were not saturated, or, in other 
words, which have preserved a residue of energy and a 
capacity of saturation which was not entirely exhausted. 
Hence they possess the power of exercising upon each 
other an action which is doubtless feeble, but sufficient 
to determine a chemicalaction. We maintain that the 
combination which has taken place, and which has 
given rise to a liberation of heat, is atomic. This heat 
could not proceed entirely from a loss of vis viva in the 
molecular motions, a loss which generally gives rise 


MOLECULAR COMBINATIONS. 249 


to physical changes, but also from a loss of vis viva in 
the intra-molecular motions—that is to say, in the 
atomic motions—which loss is the result and sign of 
chemical actions. 

But the objection will be made that this idea sup- 
poses that the molecules which we regard as complete 
are not so, and that the atoms which we consider satis- 
fied and saturated retain a residue of energy. This, 
in fact, is what must be admitted, for experience teaches 
us that it is very difficult to fix the absolute limits of 
saturation for an element, and especially a polyatomic 
element. The partisans of absolute atomicity meet with 
great difficulties when they characterise elements by the 
atomicity which is indicated by the limit of saturation 
—that is to say, by the maximum atomicity. This limit 
is not absolute, but varies with the conditions in which 
the element is placed and with the combinations 
considered. | 

Are lead and manganese saturated in their dichlo- 
rides? This is improbable, for there is reason to 
believe in the existence of tetrachlorides—very unstable, 
it is true, and which only exist in an ethereal solution 
(Nicklés), but the ephemeral existence of which never- 
theless proves that the atoms of manganese and lead 
can fix more than two atoms of chlorine. 

We see that it is impossible to fix the limits of satura- 
tion with certainty for some elements at least, and it is 
no gratuitous hypothesis to suppose that the compounds 
which appear to us saturated, and in which chemical 
forces appear to be exhausted, still retain in some of 
their atoms sufficient energy to determine combinations. 


250 THE ATOMIC THEORY. 


Such is the idea, or rather hypothesis, which may be 
brought forward to explain the existence and formation 
of so-called molecular combinations. Thus in Friedel’s 
chlorhydrate of methyl oxide we may assume that 
either the oxygen of the methyl oxide or the chlorine 
of the hydrochloric acid is still in possession of a residue 
of chemical energy. Assuming the oxygen to become 
tetravalent or the chlorine bivalent, the constitution 
of chlorhydrate of methyl oxide would then be repre- 
sented by either of the following formulz :— 


CH, .,//0l ae 


POE | 
S75 ee CH 20l=H: 


Similar considerations may be brought forward in 
order to explain the existence of a great number of 
complex combinations, double salts, and different com- 
binations containing water of crystallisation. Chemical 
force is evidently called into play in the formation of 
these combinations, for they are formed in definite pro- 
portions and with liberation of heat. But, on the other 
hand, chemists have always supposed that we had here 
to deal with a peculiar kind of chemical compound. 
The force which fixes water of crystallisation upon 
sulphate of copper might perhaps, they said, be the 
same as that which brings sulphuric acid to act upon 
oxide of copper and which maintains the elements of 
the sulphate, but it acts in amuch weaker manner. In 
fact, in many chemical actions where affinity is exhausted 
by degrees this difference in the intensity of the forces 
is manifest. In phosphorus pentachloride and penta- 
bromide two atoms of chlorine and bromine are retained 


MOLECULAR COMBINATIONS. 251 


more loosely than the other three. But how much 
more feeble the force must be which gives rise to the 
unstable combinations of phosphorus pentachloride with 
iodine chloride, or when the trichloride combines with four 
or even eight atoms of bromine, than the force which 
is called into play when phosphorus unites with three 
atoms of chlorine or bromine. The same observation 
also applies to the force which impels bromine to com- 
bine with ether to form the crystallised compound 
noticed by Schtitzenberger. We believe that it resides 
in the atoms themselves, and it seems most natural to 
attribute it, in the phosphorus compound in question, 
to the phosphorus, which can retain in the chloride 
or bromide PX, a residue of energy capable of fixing 
new atoms and of developing, if we may be allowed the 
expression, supplementary atomicities. But if, on the 
other hand, the compounds discovered by Prinvault are 
regarded as containing phosphorus, chlorine, and bromine, 
it does not appear improbable that the intervention of a 
third element should be necessary to maintain equili- 
brium between these complex and unstable mole- 
cules. One of the hypotheses by which we can make 
the theory of atomicity include all these facts consists, 
therefore, in attributing to bromine and iodine supple- 
mentary atomicities, which are developed, in some way, 
so as to unite all the atoms in the compounds in 
question. ! 


1 Thus the constitution of the compounds PCI,I and PCI,Br, 
might be represented by the following formulz :— 


bieoiéi 
Pris Ol; 


202 THE ATOMIC THEORY. 


Two series of important facts still remain to be con- 
sidered with this class of ideas—namely, the existence 
of double salts and that of compounds containing water 
of crystallisation. Can they be included in the theory 
of atomicity according to the principles just exposed 
for the ‘ molecular’ combinations of phosphorus? This 
does not appear to be impossible. Take, for example, 
the double chloride of platinum and potassium. The 
chloride PtCl, marks the limit of saturation of plati- 
num for chlorine: platinum is here quadrivalent. But 
we may suppose that it is not saturated. Though one 


Br’”—=Br, 
pre Br 
speci 


Formule analogous to the latter, in which figure several atoms of 
trivalent bromine, would explain the constitution of the compound 
PCl,Br,. These formule will appear improbable to many; I give 
them as pure hypotheses; but I beg permission to remark that 
we are here dealing with solid, unstable compounds, with crystals, 
and that the force which causes the formation of the latter is per- 
haps called into play in the aggregations of atoms: (—I=Cl,, 
—Br”=Br,). I know that here we are treading on ground crowded 
with hypotheses. I grant, on the other hand, that formule of this 
kind are easily constructed, and that the notion of atomicity, thus 
extended to molecular combinations, is very elastic. More may be 
deduced from it. The facts which we are now discussing should 
follow from it as necessary consequences, as the constitution of the 
combinations of carbon and the interpretation of their numerous 
isomers follow as a natural consequence from the notion of quadri- 
valent carbon. It must be confessed that this is not so in the pre- 
sent case. J have, nevertheless, given the preceding formule, for it 
seemed to me that the idea of referring to the atoms themselves 
all the manifestations of chemical force is worthy of attention. It 
is a stepping-stone towards a more general hypothesis, which will 
allow the rational coordination and exact representation of all the 
intermolecular forces—chemical energy, atomicity, cohesion, force 
of crystallisation, and force of solution. 


MOLECULAR COMBINATIONS, 253 


atom of platinum cannot unite with six atoms of chlo- 
rine (as osmium in OsCl,), it can unite with five atoms 
of chlorine and one of potassium and form the double 
chloride 


Cl Cl 
> Ptr ZO) 
7 PAA 

K Cl 


where it plays the part of a hexatomic or sexvalent 
element. This does not present any difficulty, for we 
merely admit a fact analogous to what we have re- 
marked in connection with ammonia and other com- 
pounds—namely, that an atom of nitrogen cannot unite 
with five atoms of hydrogen, but that it can unite with 
four atoms of hydrogen and with one atom of chlorine. 
Our considerations upon molecular equilibrium in sal 
ammoniac also apply here. 

Double salts generally contain one or more poly- 
atomic metals; in every case they contain elements 
which are or can become polyatomic and thus exchange 
supplementary atomicities with similar elements of-a 
second saline molecule (see Note I. in the Appendix). 

As to water of crystallisation, we might admit with 
Friedel that it is attached to the salts by the supple- 
mentary atomicities of the oxygen, which tends to 
become quadrivalent. But this hypothesis, the develop- 
ment of which will be found in Note II. in the Appendix, » 
we bring forward with reserve, and shall confine our- 
selves to a few short observations, upon this phenomenon 
of water of crystallisation, which is the extreme limit of 
physical and chemical actions. 

We have admitted above that when calcium chloride 


254 THE ATOMIC THEORY. 


is dissolved in water a chemical combination, properly 
so called,is formed. This salt unites, in fact, with 
water with evolution of heat, the combination remain- 
ing dissolved im the water. This is a chemical 
phenomenon ; it is independent of the physical fact of 
crystallisation and of change of state. We know, in fact, 
from the ingenious experiments of Rudorff' and Coppet ? 
upon the crystallisation of saturated solutions, that the 
combination with water of crystallisation, passing in 
some manner the point of solidification, remains in 
solution. But, independently of this chemical phenome- 
non, which, like all others, obeys the law of definite 
proportions, there may be another fact to be observed— 
namely, a physical condition, a change of state which 
intervenes—crystallisation. 

Crystalline form is undoubtedly connected with the 
atomic structure. In connection with this point we 
should notice the important work of Gaudin;? but 
before certain chemical molecules can assume certain 
crystalline forms we can well imagine that they must 
attract other molecules—water, for example, alcohol, or 
ether. And this aggregation of molecules must take 
place in definite proportions, the physical structure of 
the crystals only allowing the intervention of a definite 


' Pogg., Ann., t. cxiv. p. 63, 1861; t. cxvi. p. 55, 1862; t. cxlix. 

2 Ann. de Chim. et de Phys., t. xxiii. p. 366, 1871; t. xxv. p. 502, 
and t, xxvi. p. 98, 1872. 

8 This work would have been more remarkable and more pro- 
ductive if Gaudin, instead of devoting himself exclusively to the 
idea of symmetry in molecules, had bestowed more attention upen 
chemical considerations, as he did so successfully some time ago 
with respect to the molecular constitution of silicon chloride and 
silica. 


PROGRESSION OF ATOMICITIES. 255 


number of molecules. This is the idea which Berthollet 
applied to combinations in general. The fixed propor- 
tions, he asserted, are determined by physical conditions 
of insolubility, change of state, and crystallisation. The 
fixed proportions, we assert, in these aggregations of 
molecules, which constitute crystals, are governed by the 
physical conditions and the geometrical necessities of 
crystallisation. The phenomena which give rise to these 
molecular aggregations are therefore both chemical and 
physical in nature, and are the continuation of the 
chemical phenomena properly so called. 


Ve 


The preceding remarks show the meaning which we 
attach to the notion of atomicity. We should be over- 
looking another feature of our subject if we did not draw 
attention to the fact that the changes in the saturating 
capacity of elements—that is to say, the increase in 
atomicity—are generally found to take place in a series 
either of even numbers or of uneven numbers. With- 
out enumerating all the simple bodies, we may give the 
most striking examples of this fact. 


Elements of Even Atomicity. 


The increase of atomicity follows a series of even 
numbers in the elements belonging to the following 
groups :— 

I. Oxygen Growp.—Oxygen is bivalent. Sulphur, 


256 THE ATOMIC THEORY 


selenium, and tellurium are bivalent, quadrivalent, and 
sexvalent. 

Il. Carbon Group.—Carbon, silicon, titanium, zir- 
conium, and tin are quadrivalent, or bivalent and 
quadrivalent. 

Ill. Growps of Metals of even Atomicity.— 
Calcium, magnesium, zinc, iron, manganese: bivalent 
and quadrivalent. Chromium, molybdenum, and 
tungsten: bivalent, quadrivalent, and sexvalent. Pla- 
tinum, palladium, &c.: bivalent, quadrivalent, sexva- 
lent, and octovalent. 


Elements of Uneven Atomicity. 


The increase in atomicity follows a series of uneven 
numbers in the elements belonging to the subjoined 
groups :— 

I. Hydrogen Group.—Hydrogen, alkaline metals, 
silver, gold, and thallium: univalent and trivalent. 

II. Chlorine Group.—Chlorine, bromine, and 
iodine: univalent, trivalent, quinquivalent, and septi- 
valent. 

III. Nitrogen Group.—Nitrogen, phosphorus, arse- 
nic: trivalent and quinquivalent. Vanadium, anti- 
mony, bismuth, niobium, tantalum: trivalent and quin- 
quivalent. 

This distinction between the elements of even atomi- 
city and those of uneven atomicity is not, for some 
elements at least, without importance. Why, amongst 
so many combinations of carbon and hydrogen, do we 
meet with none which contain an uneven number of 


PROGRESSION OF ATOMICITIES. Dae 


hydrogen atoms? Because the atomicity of carbon is 
even, and the valency of its atoms in combinations is 
expressed by the numbers 2 and 4, never by the num- 
bers 1 and 3. This is the case with carbon, and 
doubtless also for other elements, though it must be 
confessed that there are exceptions to this rule. 


Nitrogen, univalent in the protoxide NO! is bi- 


valent in the dioxide NO. 

Chlorine, quadrivalent in the peroxide ClO,, 1s 
quinquivalent in chlorie acid, Cl0,(OH). 

Manganese, bivalent in MnCl, and in MnO, and 
sexvalent in potassium manganate, MnO,(OK),, is 
septivalent in the permanganate MnO,(OK). 

Tungsten, quinquivalent in the pentachloride 
WCl,, is sexvalent in the hexachloride WCI,. 

Uranium, bivalent in the dichloride UCl,, is tri- 
valent in uranyl chloride, UOCI, and quinquivalent in the 
pentachloride UCI,. 

Vanadium, AEG a3: in fie trichloride VCl,, is 
quadrivalent in vanadyl dichloride, VOCI, and quin- 
quivalent in vanadyl trichloride, VOC],. 

The consideration with which we close this chapter 
—namely, the increase in atomicities— brings us back to 
our starting point—namely, multiple proportions. They 
are fundamentally considerations upon atomicity, and are 
the same facts which formerly guided Dalton in the state- 
ment of his law, and which at the present time lead us 
to attribute to elements combining values differing with 
the form of the compound in which they occur. Thus 


the notion of atomicity follows the direct interpretation 
12 


258 THE ATOMIC THEORY. 


of facts. It rests upon a solid foundation. In its tum 
it allows us to connect, interpret, and even foresee a great 
number of facts. It is therefore useful, because it is 
productive, and we shall retain it until it is lost in a 
more general notion embracing a greater number of 
facts. 


CHAPTER III. 


CONSTITUTION OF BODIES DEDUCED FROM THE THEORY 
OF ATOMICITY. 


if 


We have endeavoured in the preceding pages to 
define the notion of atomicity, or the valency of atoms. 
It now remains to show that this notion lies at the base 
of all the partial theories which have been brought 
forward by chemists during the last fifty years, and 
especially to show how it accounts for the properties of 
those groups of atoms which we call radicals, and which 
have played so important a part in doctrines relative 
to the constitution of chemical compounds. There 
was a time when chemists could confine themselves to 
the consideration of radicals; in the written language 
of formule they were contented to represent them by 
distinct expressions, which were isolated from the other 
elements. They have now gone beyond this. Thanks 
to the indications furnished by ideas upon the satu- 
ration of atoms by each other—that is to say, by the 
theory of atomicity—they have succeeded in resolving 
these radicals, in discovering their mode of generation 
and their structure, and in determinin»y in a plausible 


260 THE ATOMIC THEORY. 


manner the connections which exist between atoms in 
combinations. This is the path which chemistry has 
recently followed, and how rapid has been the progress 
in this direction during the last twenty years! how 
many obscurities have vanished in the difficult problem 
of the intimate structure of chemical molecules, a pro- 


blem the solution of which Gerhardt declared to be im- | 


possible! and, finally, what light has been thrown upon 
the question of isomerism, which has taken such an 
important position in chemistry! We must prove this 
before concluding. 

Gerhardt’s types expressed different forms of com- 
bination (p. 211). The hydrochloric acid type repre- 
sented the combination of two univalent elements ; 
the water type, the union of a bivalent atom with two 
univalent atoms; the ammonia type, the combination 
of a trivalent atom with three univalent atoms. These 
types, then, were not taken at chance; this concep- 
tion was founded upon a profound idea, the form of 
which only has become antiquated, but which was fun- 
damentally true, and which brought to light for the first 
time the differences between the combining capacities 
of elements. The very existence of the water type 
depends upon the combining capacity of oxygen, which 
requires for saturation two univalent elements, while 
chlorine only requires one. <A single atom of oxygen 
can therefore fix not only two univalent atoms, but also 
groups of atoms which are one univalent atom short of 
saturation and the combining capacity of which is 
represented by that of this univalent atom. The number 
of these combinations, in which oxygen fixes two univa- 


0 ES a ee 


ATOMIC CONSTITUTION OF BODIES 261 


lent atoms or residues, and acts as a kind of link between 
them, is very considerable ; hence the richness of the 
water type. The same remarks apply to the ammonia 
type; it is therefore unnecessary to repeat them. We will 
merely remark that the brackets employed by Gerhardt, 
and which are still in general use, indicate that several 
elements or residues are united collectively to another 
element, an union or connection which is now expressed 
more clearly by strokes which mark the exchanges of 
units of saturation. The following symbols are there- 
fore identical :— 


Typical Formule. 


Hy See, ) 

Hf O = 3,>0 or H—-O—H 
H) H 

HON, ¢= + HON 

HJ oe 


But to return to the residues or radicals which we 
have just mentioned. We have remarked that their sub- 
stituting or combinating value is related to the state 
of saturation of the atoms. 

Thus radicals composed of carbon and hydrogen are 
derived from saturated hydrocarbons by the loss of one, 
two, three, or four atoms of hydrogen, and we have seen 
how the theory of atomicity accounts forthe state of satu- 
ration of the hydrocarbons of the series C,H,,,,(p. 213). 
These remarks may be extended to all chemical com- 
pounds. Their molecules may be considered as saturated 
when the combining capacities of their respective atoms 
are exhausted. Such molecules cannot increase by direct 
fixation of other atoms; they can only be modified by 


262 THE ATOMIC THEORY. 


substitution. But when they are deprived of atoms or 
groups of atoms representing one, two, three, or four 
valencies or units of saturation, the residues acquire a 
combining capacity or a substituting value correspond- 
ing to the loss which they have experienced. They 
become, after this loss, univalent, bivalent, trivalent, or 
quadrivalent radicals. Below are some examples :— 


Saturated Molecules. Monatomic or Univalent Radicals. 
Water, H,O —H =(OH)’ hydroxyl. 
Ammonia, NH, —H =(NH,)’ amidogen. 
Methane, CH, —H ==(CH,)’ methyl. 
Hydrocyanic acid, HNC _ —H =(CN)’ cyanogen. 
Ethane, O,H, —H ==(C,H,)’ ethyl. 
Benzene, C,H, —H =:-(C,H,) phenyl. 
Alcohol, C,H,-OH —H ==(C,H,0)’ oxethyl. 

fe __ f(C,H,-OH)’ hydrox- 

= ea oe z a ethylene. 
Acetic acid, C,H,O-OH —H =(C,H,0-O) oxacetyl. 
Ethylene bromide, C,H,Br,, —Br ==(C,H,Br)’ bromethyl. 
Antimonyl chloride, SbOCI —Cl =(Sb0O)’ antimonyl. 
Uranyl chloride, UOCI —Cl =(UrO)’ uranyl. 
Acetic acid, C,H,0O-OH —(OH)’ =(C,H,0)' acetyl. 
Benzoic acid, C,H,O-OH —(OH)’ =(C,H,0)' benzoyl. 
Nitric acid, NO,-OH —(OH) =(NO,) nitryl. 
Nitrous acid, NO-OH —(OH)’ =(NO)’ nitrosyl. 


[EGCG Ist uni- 
= valent radical of 
l glycollic acid. 
(C,H,(OHYO)Y 2nd 
Glycollic acid, C,H,(OH)/O-OH —(OHY I univalent radical 
of glycollic acid. 


Glycollic acid, C,H,(OHYO-OH —(OHY 


Boric acid, BO--OH —(OH) =(BO)’ boryl. 

Saturated Molecules. Diatomic or Bivalent Radicals. 
Ammonia, NH, —H, =(NH)” imidogen. 
Ethane, C,H, —H, =(C,H,)” ethylene, 
Ethylene bromide, C,H,Br, —Br, =(C,H,)” ethylene. 
Carbon dioxide, CO, _ —¢ > RAGES) Beemer Kear: 


bon monoxide). 


— 


4 © 


or 


‘esr’ «4+? 


a a a ee ee es ae ee ee grt ee a 


ATOMICITY OF RADICALS. 263 


Saturated Molecules. Diatomic or Bivalent Radicals. 


__ f(CO) carbonyl (car- 


Carbonyl chloride, COCI, —Cl, bertintnontiey, 
Benzene, C,H, —H, =(C,H,)” phenylene. 
Sulphuric acid, S0,(OH), —2(0H)’=(SO,)” sulpburyl. 
Oxalic acid, C,0,(0H), —2(OH)’=(C,0,)" oxalyl. 
Succinic acid, C,H,0,(0H), -——2(OH)/=(C,H,0,)” succinyl. 


f (C,H,O)” bivalent 
Glycollic acid, C,H,(OH)O-OH —2(OH)’= 4 radical of glycollic 
. acid (glycollyl). 
— J (C,H,0,)” fumaric 


Malic acid, C,H,O —H, , , 
mate 3 Land maleic acids. 
Propylic alcohol, C;,H,O —H,  =(C,H,0)” allylic alcohol. 
Saturated Molecules. Triatomic or Trivalent Radicals. 
Propane, C,H, —H, =(C,H,)” glyceryl. 
Phosphoryl chloride, POCI, —Cl, =(PO)” phosphoryl. 
Saturated Molecules. Tetratomic or Quadrivalent Radicals. 
Ethane, C,H, —H, =C,H, acetylene. 
Propane, C,H, —H, =C,H, allylene. 
Tetrane, C,H,, —H, =C,H, crotonylene. 


Some of the radicals thus formed can exist in a free 
state; others cannot be isolated, combining, when 
nascent, with each other and doubling their molecule. 
The monatemic radicals which we have enumerated above 
must all be included in the latter category; not one 
exists in a free state. Hydroxyl, OH, does not exist; 
combined with itself it constitutes hydrogen peroxide, 
H,O,=HO-OH. All attempts have as yet been unsuc- 
cessful to isolate double amidogen, N,H,=H,N-—NH,, 
but substitution derivatives of this body are 
known. It is well known that free cyanogen contains 
C,N,=NC-CN=2 volumes. As to methyl, the 
moment it is separated from iodine—for example, in 
methyl iodide—it doubles its molecule to form free. 


264 THE ATOMIC THEORY. 


methyl or ethane, C,H,=H,C-CH,=2 volumes. The 
case is the same with ethyl, phenyl, and generally with 
all the radicals of saturated monatomic alcohols. The 
radicals of monobasic acids, such as acetyl and benzoyl, 
neither exist free nor in a state of double combination. 
On the other hand, Brodie has described, as peroxides 
of acetyl and benzoyl, compounds which may be regarded 
as resulting from the union of two molecules of oxacetyl 
or oxybenzoyl. 


(C,H,0-0), peroxide of acetyl. 
(C,H,O-O), peroxide of benzoyl. 


As regards diatomic radicals, a great number of them 
exist in a free state. This is the case with ethylene 
and its homologues, with carbon monoxide or carbonyl, 
with sulphurous acid gas or sulphuryl, with nitrogen per- 
oxide or nitryl, with nitrogen dioxide or nitrosyl. The 
tetratomic radicals, acetylene, allylene, and crotonylene, 
are known ina free state. We observe that all these 
bodies behave exactly like radicals in the sense which 
was formerly attached to this term; for are they not 
capable of combining directly with simple bodies, such 
as chlorine? The chlorides of carbonyl, sulphury], 
ethylene, and acetylene are formed by the direct union 
of chlorine with the isolated radicals, just as the metal- 
lic chlorides are formed by the direct union of chlorine 
with a metal. And ethylene fixes two atoms of chlorine, 
because its saturation is incomplete by two atoms of 
hydrogen. As to the oxygenated radicals, carbonyl and 
sulphuryl, they can fix not only two atoms of chlorine, 
but even an atom of oxygen to form the carbonic and 


CONSTITUTION OF COMPOUNDS. 265 


sulphuric anhydrides CO, and SO,. All this is too 
simple and too well known to require further remark. 
We will only add that the triatomic radicals containing 
carbon, such as glyceryl, (C,H,), do not exist in a free 
state. Free allyl or diallyl, (C,H,),, bas doubled its 
molecule. 

These are facts which we have just discussed. We 
must now show how they are accounted for by theory. 

Carbon of even atomicity is united in all its com- 
binations to a number of elements representing an even 
sum of units of saturation. This is why the groups 
CH, and CN do not exist separately. In these groups 
the three atoms of hydrogen and the atom of nitrogen 
represent an uneven number of units of saturation. 
They both require one atom of hydrogen to form the 
saturated combinations CH, and CNH; their combin- 
ing or substituting value is equal, therefore, to that of 
one atom of hydrogen, and where an atom of hydrogen is 
wanting they can fill its place. They can also, by com- 
bining together, supply their mutual deficiency. In this 
manner are formed the compounds H,C—CH, free 
methyl, NC-CN free cyanogen, H,C-CN methyl cya- 
nide. But it isan important fact that the combination of 
these groups with each other is accomplished by the car- 
bon atoms. These are the atoms which are not saturated 
in their affinities, which are mutually impelled towards 
each other in order to satisfy them. This is the new and 
essential point. The properties of the radicals are referred 
to the atoms themselves. Formerly they were considered 
asa whole. To the radical regarded as a whole was at- 
tributed the power of combining with or of being sub-_ 


266 THE ATOMIC THEORY. 


stituted for simple bodies. This was the fundamental 
point of view of Gerhardt’s theory of types. We now go 
further. To discover and define the properties of radi- 
cals we go back to the atoms of which they are composed, 
and thus substitute a general hypothesis for a particular 
theory—namely, that the atomicity of radicals is sub- 
ordinate to the atomicity of the elements. Thus methyl 
and cyanogen are monatomic or univalent radicals, 
because they contain the quadrivalent element carbon, 
which is not saturated. And this is the case also with 
radicals of higher atomicity. Why does carbonic mon- 
oxide act as a diatomic radical? Because the carbon 
which it contains is not saturated. Why, again, does 
ethylene, C,H,, fix two atoms of chlorine or bromine, 
thus acting as a diatomic radical? Because both the atoms 
of carbon which it contains are unsaturated ; both can 
therefore directly fix other atoms, without breaking the 
link by which the two carbon atoms are riveted together. 

But we must look more closely into the matter. 
Everything leads us to admit that in ethylene, C,H,, 
the two carbon atoms each attract two atoms of 
hydrogen, and that these two atoms of carbon, both of 
which are quadrivalent, can only satisfy the combining 
capacity residing in them by mutually exchanging two 
units of saturation, after having both fixed two atoms 
of hydrogen. We are thus forced to regard ethylene, 
and consequently analogous hydrocarbons, as containing 
two carbon atoms united by a double link—that is to say, 
by a double exchange of units of saturation—and this 
is the manner in which we must understand the formula 
H,C = CH,=C2H,, by which a great number of chemists 


CONSTITUTION OF COMPOUNDS. 267 


represent the constitution of ethylene. But it may be 
objected that this is pure fiction, and that it would be 
simpler to admit that in ethylene carbon plays the part 
of a trivalent element, H,C—CH,, the two carbon atoms 
being united by a single exchange of atomicities. It is 
not a fiction, it is in accordance with facts, for we must 
not forget that all known hydrocarbons contain an 
equal number of hydrogen atoms. This would not be 
the case if carbon could play the part of a triatomic or 
trivalent element; if so, methyl, CH,, and ethyl, C,H,, 
should exist in a free state. We must, therefore, con- 
clude, taking experiment as our authority, that in the 
combinations of carbon and hydrogen carbon is never 
bivalent, as it is in carbon monoxide; methylene, CH.,, 
does not exist: that it is never trivalent ; methyl, CH,, 
and ethyl, C,H,, do not exist. It is therefore quadriva- 
lent or tetratomic, and we are thus led to admit that 
carbon atoms have the faculty of exchanging with each 
other several units of saturation. But the combination 
thus constituted is in a state of unstable equilibrium, 
which is destroyed by the intervention of chlorine. The 
latter can fix itself upon molecules so formed, thus 
destroying the double link and constituting a perfectly 
saturated molecule. It is true that the affinity of carbon 
for carbon is strong; but when two atoms of this simple 
body have exchanged two units of saturation this 
affinity cannot stand, as far as the second bond is 
concerned, against that of chlorine, which tends to fix 
itself on to both the groups CH,. The following 
formule will explain this view of the case :— 


C,H, =H,C=CH, ethylene; 
©,H,Cl,—ClH,C-CH,Cl ethylene chloride. 


268 THE ATOMIC THEORY. 


Ethylene and analogous radicals have, therefore, the 
power of directly fixing chlorine and other elements, 
because they contain atoms of carbon, the combining 
capacity of which is not exhausted; since it is twice 
exerted between carbon atoms, it can still be manifested 
towards the atoms of chlorine. The latter severally fix 
themselves upon an atom of carbon in olefiant gas, 
although they refuse to unite directly with free carbon ; 
the affinities of this body are, in fact, very different 
according as it is considered in the state of a simple 
compact and condensed body, C,, or in a state of com- 
bination with hydrogen and in a gaseous form.! 


1 It is a circumstance worthy of remark that chlorine or bromine, 
which do not unite with free carbon, as does oxygen, can fix them- 
selves directly upon the unsaturated hydrocarbons, which oxygen 
cannot do. Are we to conclude that the chlorine is attracted, not 
by the carbon, but by the entire ethylene group acting as a radical, 
as was formerly supposed? This would be going a step backwards. 
In my opinion it is unquestionably the unsaturated carbon which 
attracts or admits the chlorine: it attracts it because it occurs in 
gaseous combination with hydrogen; it admits it because there are 
two vacant places in the system. The saturated hydrocarbon C,H, 
also attracts chlorine ; but, as there is no vacant place in the system. 
it can only admit it by losing twoatoms of hydrogen. The hydrogen 
atoms seem, therefore, to exercise an influence upon the property 
possessed by carbon of fixing chlorine—that is to say, of admitting 
this element into its sphere of action. Such an influence is exer- 
cised in other instances and by other elements. Ethylene, which fixes 
chlorine, is incapable of directly fixing oxygen, but dibromethylene, 
C,H,Br,, can fix it, according to Demole, to form the compound 
C,H,BrO-Br (bromacetyl bromide). In this case, as in the former, 
the affinities of carbon have been modified by the intervention of 
other elements—hydrogen or bromine. 


CONSTITUTION OF COMPOUNDS. 269 


i 


The foregoing considerations upon the hydrocarbon 
radicals apply to all compounds capable of directly 
fixing elements, which compounds, in virtue of this 
property, resemble radicals. These elements are 
attracted by one or other of the unsaturated atoms 
contained by the compound in question. Let us take 
some examples. 

Carbon monoxide can directly fix oxygen or chlorine 
because the bivalent carbon which it contains is not 
saturated. In carbonyl chloride and carbon dioxide 
the carbon has become quadrivalent. Like carbon 
monoxide, sulphurous acid gas can fix oxygen or chlorine, 
and it is the sulphur which attracts these elements. In 
sulphuryl chloride and in anhydrous sulphuric acid the 
sulphur has become sexvalent.' 

Phosphorus trichloride, in fixing directly two atoms 
of chlorine, behaves, in some respects, like a radical, 
and it owes this property to the unsaturated phosphorus 


1 Sulphurous acid gas being S'"0, or =S“0,, sulphuryl chloride is 


Ch. 79 HOW me 
Qs" Xo” sulphuric acid HO/» XO" 


OCI : : 
The formula BiG ser which might be attributed to sulphuryl 


chloride, does not seem to us probable, since oxygen does not possess 
any tendency to unite with chlorine, and because the properties of 
sulphuryl chloride are not those of a body containing the hypo- 
chlorous residue OCI, which would decompose with explosion. We 
may add that selenium and tellurium are manifestly quadrivalent 
in the tetrachlorides and sexvalent in the anhydrides and selenie 
and telluric acids. : 


270 THE ATOMIC THEORY. 


which it contains. The difference between the two 
views of the mode of action displayed by radicals is 
here shown in a most striking manner. It was 
formerly maintained that phosphorus pentachloride 
should be regarded as a combination of phosphorus tri- 
chloride with chlorine; the trichloride exists in it as a 
whole, as a radical endowed, as such, with a power of 
combination. We say now that the trichloride can 
take up chlorine because the phosphorus which it 
contains is not saturated ; in the pentachloride phos- 
phorus is united directly with five atoms of chlorine, 
and when phosphorus trichloride takes up two atoms 
of chlorine the latter are attracted by the unsaturated 
atom of phosphorus. When phosphene fixes hydriodic 
acid, or when ammonia unites directly with hydro- 
chloric acid, they also act as radicals, and owe this 
property to the atom of phosphorus or of nitrogen 
which they contain, both of which show a tendency to 
become further saturated. In hydriodate of phosphene 
(phosphonium iodide), as in hydrochlorate of ammonia 
(ammonium chloride), they become quinquivalent. 

In organo-metallic radicals properly so called we 
find properties of the same order, which we interpret in 
the same manner. And it must be confessed that these 
ideas upon the saturating capacity of elements, a 
capacity varying with the combinations in which they 
occur, are the natural consequence of the experiments 
undertaken twenty years ago upon the class of com- 
pounds in question. We refer to the classical dis- 
coveries of Frankland, Baeyer, Cahours, and the 
ingenious views which they introduced into science, 


4 


CONSTITUTION OF BODIES. 271. 


When Frankland compared with each other stannic 
iodide, stannethy! iodide, and stannic ethide, expressing 
the composition of these bodies by the formule 
Sn 17 Sn { rs Sn { ony 

surely he showed by this notation that in these three 
bodies iodine and ethyl are combined in the same 
manner with tin, and that stannethyl, SnC,H,, only 
plays the part of radical because the tin which it 
contains tends to pass into the state in which it exists 
in stannic iodide. Stannethyl, SnEt, has just as much 
claim to be considered as a radical as stannous iodide, 
and in both cases it is the tin itself, and not the 
radical considered as a whole, which attracts the iodine. 

And in his masterly statement of the theory of ‘the 
saturation in the organo-metallic compounds of tin 
Cahours referred the power of attracting either chlorine, 
methyl, or ethyl, in order to attain a stable molecular 
equilibrium, to the tin itself, so that the general com- 
position of all these saturated compounds might be 
expressed by the formula 

snX,.? 


1 These formule are on the old notation: C=6, Sn=59. 

2 Cahours wrote Sn,X,. Withthe atomic weight of tin, Sn=118, 
this expression becomes SnX,, and the saturated compounds of tin 
receive, in consequence, the following formule :— 


SnCl, = 2 vol. stannic chloride. 
SnEt, =2 vol. stannic ethide. 
SnMe, =2 vol. stannic methide. 


SnEt,Me,=2 vol. stannic dietho-dimethide. 
SnEtMe, =2 vol. stannic etho-trimethide. 
SnEt,Me =2 vol. stannic trietho-methide. 


272 THE ATOMIC THEORY. 


The condition of tin in these compounds differs, 
therefore, from that which it occupies in the stannous 
compounds SnX, ;! in the latter case it can attract ele- 
ments or groups representing a combining value X,, 
which it is incapable of doing in its saturated com- 
pounds. 

The same remarks apply to arsenic in its methyl 
compounds; they belong to the two types AsX, and 
AsX,. Now, Baeyer proved as early as 1858 that in the 
compounds of both series arsenic is united in the same 
manner to methyl and chlorine. 


Type AsX,. Type AsX,. 
AsMe,Cl tetramethyl-arsonium chloride. AsMe,  trimethylarsine. 
AsMe,Cl, trimethylarsine dichloride. AsMe,Cl dimethylarsine 
monochiloride. 
AsMe,Cl, dimethylarsine trichloride. AsMeCl, monomethylar- 


sine dichloride. 

AsMeCl, monomethylarsine tetrachloride. AsCl, oes trichlo- 
This eminent chemist showed that the methyl com- 
pounds belonging to the type AsX, can directly fix Cl, 
and play the part of radicals. We say that this is the 
case because arsenic, trivalent in these compounds, tends 
to become quinquivalent in those in which it is saturated. 
And we see at once from this example that the limits of 
saturation are variable for each element, and subordinate 
to the nature of the simple bodies or groups with which 


SnEt,Cl =2 vol. stannic chloro-triethide (chloride of sesquistann- 
ethyl). 
SnEt,l  =2 vol. stanniciodo-triethide (iodide of sesquistannethyl). 
SnMe,I, =2 vol. stannic diodo-dimethide (iodide of stanno-dime- 
thyl). 
? Or 2SnX,,. 


Te 


CONSTITUTION OF COMPOUNDS. 273 


this element is combined. Arsenic is saturated with 
chlorine in the trichloride as it is saturated with 
hydrogen in the trihydride. It is no longer saturated 
in the methyl compounds corresponding to the trichloride. 
The compounds AsMeCl,, AsMe,Cl, AsMe,, can directly 
fix Cl,. 

There is, we think, no need to add further examples 
to those which we have already given. We have 
demonstrated and proved the fundamental point which 
we wished to bring forward—namely, that the pro- 
perties of radicals capable of fixing other elements, 
after the manner of simple bodies, must be referred 
to the properties of the atoms contained in these 
radicals. 

The same view must be extended to the residues or 
remains of various atomicity, which cannot be isolated 
as such, but of which the individual existence is 
admitted by the theory of radicals and types in organic 
compounds and in a great number of mineral com- 
pounds. Methyl, CH,, ethyl, C,H,, acetyl, C,H,0, 
glyceryl, C,H, (allyl), and many other analogous 
radicals do not exist; when we try to liberate them, 
they destroy themselves in combining with each other 
and doubling their molecule. The reason of this is that 
these radicals contain carbon, which is always of even 
atomicity ; one of their carbon atoms, being combined 
with a sum of elements representing an uneven number 
of units of saturation, tends to complete this saturation. 
Thus CH, unites with H, or with Cl, or with OH, or 
with N H, or with CH,, to form the following saturated 
compounds :— 


Q74 THE ATOMIC THEORY. 


CH,H marsh gas (methyl hydride), 
CH,Cl methyl chloride, 

CH,(OH)’ methyl hydrate, 

CH,(NH,)’ methylamine, 

CH,(CH,)’ free methyl or ethane. 

In marsh gas the four atoms of hydrogen are united 
to the carbon in the same manner, and if we give to 
this body the name of methyl hydride it is simply to 
show that one of these hydrogen atoms—it matters little 
which—can be replaced by a chlorine atom or bya 
monatomic group. In methyl hydrate, H,C—O”H, the 
carbon atom completes its saturation by uniting with 
an atom of oxygen; but, as the latter is bivalent, one 
atomicity remains free or disposable; it is satisfied by 
an atom of hydrogen. The same view may be taken of 
methylamine, H,C—N’’H,, in which the nitrogen 
saturates by one of its atomicities the carbon atom » 
of the methyl, and by two others the two atoms of 
hydrogen. The two latter are not directly connected 
with the atom of carbon; they are united to the nitro- 
gen atom, and, as it were, attached to it. In methyl 
oxide, H,C—O”’—CH,, the bivalent oxygen completes 
the saturation of the two carbon atoms of two methy- 
lic groups. In free methyl or ethane, H,C—CH,, 
the two carbon atoms reciprocally complete their satu- 
ration. 

Such is the new conception of radicals, a conecption 
founded upon the atomicity of elements, and which is 
translated into notation by the preceding formule, 
the signification of which must now be intelligible to 
all. These formulze, founded upon the theory of the 
reciprocal saturation of elements, allow the demonstra- 


CONSTITUTION OF COMPOUNDS. 275 


tion of the molecular structure of methyl compounds ; 
they indicate the relations which exist between the 
several atoms composing the molecule ; they also express 
the mode of generation and the properties of the com- 
pounds in question. 

In methyl hydrate, methyl oxide, and in methyla- 
mine the molecule is easily broken up: the oxygen and 
nitrogen are again separated from the carbon, the 
methyl group passing by exchange into other compounds. 
Thus, for example, hydrochloric acid easily converts 
methyl hydrate into methyl chloride, with formation of 
water. We have here a double decomposition, in which 
the radical passes intact from the hydrate into the 
chloride. This facility of exchange, which is exhibited 
| by radicals in an immense number of reactions, was ex- 
pressed most clearly by the typical notation. But it is 
a remarkable fact that this property does not extend 
to free methyl or ethane, since the carbon is firmly 
riveted to carbon, which explains, on modern ideas, 
the relative stability of this compound, a fact which 
caused so much surprise to chemists twenty-five years 
ago. 

Let us go a step further in this direction, and now 
consider a radical containing two atoms of carbon—ethyl, 
C,H,. This is ethane minus anatom of hydrogen: one 
of the carbon atoms is united with only two atoms of 
hydrogen. It is the latter which is not saturated and 
which, to complete its saturation, can attract, as in the 
preceding case, an atom of hydrogen, chlorine, gee 
or a monatomic group. 


276 - THE ATOMIC THEORY. 


CH,’ CH, CH, CH, CH, CH, CH, CH, CH, 


| | | | | 
Utes CH, ~CH, CH. Ui.—_O—CH. “CH,” CHS) Ure 
| | 


| | | | I | 
he Aa OH NH, CN CH, 

Ethyl. Ethyl Ethyl Ethyl Ethyl oxide. Ethyla- Ethyl Ethyl 
chloride. iodide. hydrate. mine. cyanide. inethylide 
(propane). 


In all these compounds we notice a part which is 
common to all, forming as it were the solid nucleus of the 
molecule, upon which are riveted, and as it were grafted, 
the various appendices. And in a great number of 
reactions this molecule will break up in such a manner 
that the appendices alone are removed, the nucleus 
remaining intact, and passing by exchange into another 
combination ; one ethyl compound is thus converted into 


another ethyl compound. This, however, is not always | 


the case. Ethyl iodide and ethyl cyanide, though analo- 
gous compounds, do not behave in the same manner 
under the influence of potash: the former yields alcohol 
by double decomposition; the latter propionic acid, a 
compound containing three atoms of carbon, like the 
cyanide itself.! Thus iodine is easily separated from the 
atom of carbon to which it 1s united, whilst the carbon 
atom of the cyanogen refuses to be separated. In this 
case it isas strongly united to carbon as the carbon atom 
of methyl in ethyl methide or propane. The strong 
affinity of carbon for carbon accounts for the diversity of 
these reactions, which was an enigma in the old theory 


1 CH, CH, CH, CH, 


| | 
CH,+2H,0 = CH, +NH, 


| | 
CH,+ KOH = CH,+KI 


| 
OH 


I CN CO—OH 
Ethyl iodide. Ethyl hydrate Ethyl cyanide. Propion‘c acid. 
(alcohol). 


CONSTITUTION OF COMPOUNDS. 277 


of radicals. This clearly shows the difference between 
the two points of view. In the old conception we have 
a group of atoms considered as a whole and united as 
such either to iodine or to cyanogen: in the modern 
conception, a group of atoms constituted in a certain 
manner and containing @ certain atom of carbon united 
either to iodine or tocyanogen. Inthe former case the 
radical acts as a whole; in the latter it is resolved into 
different elements and exhibits its activity by one of its 
carbon atoms, which is not saturated. 

The case which we have just described is general. 
Hydrocarbon radicals analogous to ethyl can be resolved 
as we have just resolved this radical, the carbon atoms 
being united together and the hydrogen atoms unequally 
distributed amongst the carbon atoms. One of the 
latter will contain one less than it requires for satura- 
tion: this is the atom upon which other univalent | 
elements or groups can fix themselves, by which the 
hydrocarbon group as a whole will act as a radical. 

These ideas are applicable to the oxygen radicals 
derived from the former. Acetic acid contains such a 
radical: acetyl is ethyl modified by substitution. 


CH, CH, 
| 
sO —O0" 
Ethyl. Acetyl. 


In this oxygenised radical the atom of carbon com- 
bined with the oxygen appears in the same state of 
saturation as in ethyl, where it is united with two atoms 
of hydrogen. It is this carbon atom which fixes chlo- 
rine in acetyl chloride, hydrogen in aldehyde, hydroxyl 


278 THE ATOMIC THEORY. 


in acetic acid, the group NH, in acetamide, and oxygen 
in acetic anhydride. 
oe CH OH: CH, CH, CH, 
| 
COCl COH CO—(OH)’ CO—(NH,)’ CO—O”—OC 


Acetyl Aldehyde. Acetic acid. Acetamide, Acetic anhydride. 
chloride. 


ids 


We have now made an important step, by showing 
how radicals can be resolved into definite atomic groups, 
in which we try to establish, by means of considerations 
founded on atomicity, the relations which exist between 
the atoms, the group acting as a radical whenever one 
or other of these atoms exists in an unsaturated state. 
It is evident that the formule which are developed 
according to the principles just explained, and which 
naturally increase in complication with the number of 
carbon atoms contained in the organic compound, do not 
represent the position of the atoms in space; being 
represented upon one plane, they cannot represent the 
form of a molecule which would occupy three dimen- 
sions in space. They only indicate, therefore, connections 
or, if you will, relations of juxtaposition. And these 
indications, incomplete though they are, are invaluable 
in a great number of cases. We must add, however, 
that attempts have been made to go a step further by 
forming hypotheses upon the geometrical structure of 
certain molecules and upon the probable grouping of 
atoms in space. We shall say a few words upon ‘this 
presently. | | 


CONSTITUTION OF COMPOUNDS. 279 


Radicals containing carbon are by far the most 
numerous in chemistry; they are not, however, the 
only ones, and it has long been impossible to consider as 
correct Liebig’s definition—organic chemistry is the 
chemistry of compound radicals. The principles which 
we have just discussed may be applied to all groups 
acting as radicals, some of which we have enumerated on 
p. 262. Whydoes phosphoryl, the existence of which 
has been admitted in phosphoryl chloride, PO.Cl,, and 
in orthophosphoric acid, PO.(OH),, play the part of a 
trivalent radical? Because it contains quinquivalent 
phosphorus. The latter has, by its union with oxygen, 
lost only two units of saturation ; it has, therefore, three 
more to dispose of in some way, and these are represented 
in the chloride by three atoms of chlorine, and in phos- 
phoric acid by three hydroxyl groups. In these bodies the 
single atom of phosphorus is therefore in connection 
(1st) with an atom of oxygen, (2ndly) with three atoms 
of chlorine or with the oxygen atoms of the groups OH. 
The formule 


express, therefore, the connections which exist between 
the different atoms in phosphorus oxychloride and in 
phosphoric acid. The latter enables us to conceive what 
takes place when phosphoric acid is dehydrated ; in fact, 
the atomic constitution of pyrophosphoric acid and of 
metaphosphoric acid follows naturally from that of ortho- 
phosphoric acid. This point having been already 
developed in the note upon p. 243, we need not return 


280 THE ATOMIC THEORY. 


toit here. We will merely remark that as far as pyro- 
phosphoric acid, 

O=P(0H), 

O 

oSP(OH),, 
is concerned, the intermediary atom of oxygen is con- 
nected with both the atoms of phosphorus. It is by 
this atom of oxygen that the two residues of the 
two partially dehydrated molecules of phosphoric acid 
are united to each other. It here plays the same part 
as the atom of oxygen in ethyl oxide (p. 276), where 
this atom rivets together the two ethyl groups, because 
itis united to the carbon of each group. We here see, 
from a striking example, with what facility the theories 
arising from an intelligent study of organic compounds 
may be applied to mineral compounds alsuv. In this 
lies the character and the advantage of the theory of 
atomicity. It has cemented the alliance between in- 
organic and organic chemistry. It has given the key 
to the theory of radicals; it adapts itself perfectly to 
that of types; it binds together these two theories by 
subjecting them to a more general idea. 

In the developed formule which we have given 
above the radicals are no longer represented as distinct 
groups, as if they had an individual existence. Such 
formule are acceptable from a theoretical point of 
view, and useful in a great number of cases.) We must 
consider that, properly speaking, radicals do not exist 
as such in compounds. This is clearly demonstrated 
by the new notation, which shows that chemical com- 
pounds form a whole, and enables us to interpret a large 


CONSTITUTION OF COMPOUNDS. 281 


number of reactions in which this whole, this molecular 
edifice, undergoes a more or less violent change. As 
soon as reactions become complicated Gerhardt’s no- 
tation is no longer of service. It represented radicals 
by single expressions; it thus enabled us to represent 
in a striking manner reactions in which they are ex- 
changed by double decomposition; it gave no informa- 
tion about those in which they are destroyed. Do we by 
this mean to say that we should entirely abandon this 
notation, so remarkable for its simplicity, for teach- 
ing purposes ? By no means; condensed formule possess 
great clearness, and there are abundant reasons for. 
their maintenance. Why should we not write alcohol 
C,H,.OH, ether (C,H,),0, ethylamine C,H,.NH,, glycol 
C,H,(OH),, glycerine C,H,(OH),, nitric and metaphos- 
phoric acids NO,.OH and PO,.OH, and orthophosphoric 
acid PO(OH),? These formule are simple and sufficient 
in agreat number of cases. They represent a certain 
number of reactions of which they are in a way the 
reflection. Is it not simpler and more convenient to 
represent amylic alcohol by the formula C,H,,.0H, 
than by a formula developed in the following manner? 
CH,—CH,—CH,—CH,—CH,—OH. 
Amylic alcohol. 

This formula, which represents normal amylic 
alcohol, only becomes useful when it is necessary to dis- 
tinguish this alcohol from its isomers (see the note, 
p- 336). In all other respects the condensed formula 
stands good. We are here considering a relatively 
simple body ; but if it were necessary to express by a 
developed formula the composition of ethal, C,,H,,0, or 


13 


282 THE ATOMIC THEORY. 


of stearic acid, C,,H;,0,, or of cerotic acid, C,,H,,0,, a 
whole page would not suffice. It is inconvenient in 
practice, for the eye has some difficulty in at once 
grasping these expressions, so widely spread upon the 
paper. It is, however, necessary to practise and to be- 
come accustomed to this exercise, for it is only by these 
formule that we gain an idea of the constitution of 
bodies, of molecular grouping, and cases of isomerism, 
facts which are essentially within the sphere of chemis- 
try ; for the properties of bodies and their reactions 
are unquestionably a function of this molecular group- 
ing. This statement has often been made, and is in 
some sense a commonplace truth ; but how great is the 
distance between this statement and even the demon- 
stration which has been attempted within the last few 
years, The task was a difficult one, and had baffled the 
most ingenious and the most intelligent chemists with 
Gerhardt at their head. We have seen the means by 
which it has been possible to attack it, and how con- 
siderations relative to atomicity have led to the forma- 
tion of hypotheses upon the grouping of atoms. That 
they are hypotheses must not be forgotten. Some are 
good, some are uncertain; and, in considering the 
formulz by which we endeavour to represent the con- 
stitution of bodies, those only should be accepted which 
are the direct and established expression of facts: all 
others are without value. This is an important point ; 
we shall, therefore, now endeavour to make it clear by 
pointing out the mode of construction, the exact mean- 
ing, and the real utility of the developed formule in 
question. 


RATIONAL FORMULA. 283 


De 


We will first consider a relatively simple compound, 
elycerine, which only contains three atoms of carbon. 
These three atoms of carbon are united together, form- 
ing a group or nucleus which we meet with again not 
only in all the direct derivatives of glycerine, but also 
in the products of its decomposition or transformation, 
propylene, allyl alcohol, and acrolein. The constitu- 
tion of the carbon nucleus of glycerine is represented 
in the developed notation by a chain of three carbon 
atoms riveted together. The atoms of hydrogen and 
oxygen are distributed among these carbon atoms in 
such a manner that each atom of carbon exchanges with 
the neighbouring atoms four units of saturation, and 
each atom of oxygen two. This is shown in the 
formula 

CH,.0OH 
da.on 
CH,.OH. 


Another method of distribution of the atoms of 
oxygen and hydrogen between the carbon atoms is pos 
sible; but the number of these possible atomic arrange- 
ments is necessarily limited, from the fact that they 
must all satisfy the double condition that carbon is 
quadrivalent and oxygen bivalent. As examples of 
bodies possessing the composition of glycerine and a 
different atomic arrangement we may quote the fol- 
lowing :— 


284 THE ATOMIC THEORY. 


CH.(OH), CH, 
CH, C(OH,) 
CH,OH .  CH,.OF 


These bodies would be isomers of glycerine, the 
existence of which is foreseen by theory. They are 
unknown, and we must add that there is little probability 
of their existence in a state of liberty, because those 
compourds in which one atom of carbon is united to 
two OH (hydroxyl) groups have but little stability.! 

The above construction of the formula of glycerine 
accounts for the properties and transformations of this 
body. Let us take two examples. Nothing is easier 
than to represent the transformation of glycerine into 
allyl alcohol and into trichlorhydrin. 

The first reaction is expressed by the following 
equation :— 


CH,.0H CH, 
I 

CH.OH + H, = CH + 2H,0 
| | : 
CH,.0H CH,—-OH 
Glycerine. - Allyl alcohol. 


But allyl alcohol is not a saturated compound; it 
is capable of directly fixing bromine, hydrobromic acid, 
&e. The preceding formula expresses this fact, by 
showing that two atoms of carbon exchange with each 
other two units of saturation and are riveted together 


CCl 
1 Thus chloral hydrate | ; is easily decomposed inte 
C 2 


CCl, 
chloral | and water. 
CHO 


RATIONAL FORMULA. 285 


bya double bond. This is indicated by the double 
connecting bond. It is suppressed in the additive 
product 

CH,B 

CH.B 

baie 


dibromopropy! alcohol (allyl-aleohol bromide), which is 
the result of the direct action of bromine upon allyl 
alcohol. 

When glycerine is transformed into trichlorhydrin 
by the successive action of hydrochloric acid and phos- 
phorus pentachloride, the three OH groups are replaced 
by three atoms of chlorine — | 


CH,.0H CH,.Cl 


| 
CH.OH + 3HCl = 3H,0 + CH.Cl 


CH,.OH CH,.Cl 
Glycerine. Trichlorhydrin. 

We see that in trichlorhydrin each atom of carbon 
is connected with an atom of chlorine, and this distribu- 
tion of the chlorine atoms is characteristic of trichlor- 
hydrin. A different distribution would imply a different 
atomic arrangement, and bodies thus constituted, though 
formed of the same atoms as trichlorhydrin, and possess- 
ing, consequently, the same composition and the same 
general formula, will be isomeric and not identical with 
trichlorhydrin.. These isomers exist. Their existence 
is foreseen, their number limited, and their constitution 
indicated by theory. This must be prov ed, for it is the 
whole point of the question. 

Given three atoms of carbon united together and 


286 THE ATOMIC THEORY. 


possessing eight units of saturation, we proceed to 
distribute amongst these three carbon atoms five atoms 
of hydrogen and three atoms of chlorine, in such a 
manner as to attribute to each carbon atom four valen- 
cies or units of saturation. There are five different ways 
of satisfying this condition, and, consequently, theory 
can predict the existence of only five bodies presenting 
the composition of trichlorhydrin, Their molecular 
structure is expressed by the following formulze :— 


I. 1. III. Iv. v. 
CH,Cl CHCl, CHCl, CCl, CH,Cl! 
| | | | 
CHCl CEC CH, CH, CCl, 
ene | | | 
CH,Cl CH, CH,Cl CH, CH, 
Trichlor- Boiling point 140°. Unknown, Unknown. Metbyl-chlor- 
hydrin ; acetol cblor- 
boiling ide ; boiling 

point point 123°. 


158°, 
Two of these bodies have not yet been obtained ; 

but we have learnt from frequent experience that gaps 

of this kind may be filled up, and we could quote cases 


CH, 
1 The hydrocarbon CH,, the type of combination from which all 


CH, 
these chlorine compounds are derived, possessing a symmetrical 
structure, the number of these derivatives is limited to five, because 
the substitutions effected in one of the groups CH, are equivalent to 
those effected in another group CH,. 
Thus the following chlorine derivatives must be regarded as 
identical :— 


CHCl, CH, CCl, CH, 


ion identical Eas ee identical We 


| with | | 2 with 
CH, CHCl, CH, CCl, 


ISOMERISM. 287 


more complicated than that under discussion where the 
attempt has been most successful ;' and it is an im- 
portant fact that, if we except cases of purely physi- 
cal isomerism and those in which dimorphism comes 
into play, no case of chemical isomerism has as yet 
been observed which would not agree with those pre- 
dicted by theory. We have here a striking confirma- 
tion of the theory which limits the number of possible 
isomers by considerations relative to atomicity or to the 
reciprocal saturation of atoms. Without these considera- 
tions a far larger number of isomers might be deemed 
possible. 

Thus, to return to the preceding case, given three 
atoms of carbon, five atoms of hydrogen, and three 
atoms of chlorine, if it were merely a question of 
distributing the atoms of hydrogen and chlorine 
amongst the atoms of carbon, the numbers of possible 
arrangements between these different would be very 
considerable, and easily calculated by the rules of 
algebra. 

A celebrated chemist, Berthelot, has been guided by 
peculiar considerations in theoretical predictions relative 
to the number of possible isomers in a given case. He 
admits that the same body may yield different isomers, 


CH,Cl CH, 


haere tact 
Co, itn OC 
CH, CH,Cl. 


The identity of the bodies in question will at once be seen when we 
observe that their formule are simply reversed. 

1 The isomers of amylene will be mentioned presently, and in 
Note III. (p. 336) the isomers of amyl alcohol. 


288 THE ATOMIC THEORY. 


according to the manner in which it has been formed. 
Thus propyl hydride or prepane may be formed in 
different ways—namely, by the addition of two atoms 
of hydrogen to propylene, or by the addition of methane 
to ethylene, or again by the addition of methane to 
ethane with a loss of two atoms of hydrogen.’ If now 
in this hydrocarbon, propane, three atoms of chlorine 
are substituted for three atoms of hydrogen, the com- 
pounds formed may differ, in the first place, according 
to the manner in which the propane acted upon has 
been formed; in point of fact, says Berthelot, pro- 
pane may contain different residues, according to the 
nature of the generating hydrocarbon. In the second 
place, it may yield distinct isomers, according to the 
part which is played in it by these residues. This point, 
again, may produce differences in the trichlorinated 
derivatives. They may differ, finally, according to the 
order in which the substitution has been effected. 
Thus, supposing three atoms of hydrogen to be suc- 
cessively replaced by three atoms of chlorine, the tri- 
chlorinated compounds thus formed may differ according 
to the order of succession in which these hydrogen atoms 
have been replaced. 

Several objections may be raised against these theo- 
retical views. In the first place, we cannot admit that 
a difference in the mode of formation should be a suffi- 
cient reason to determine isomers. In fact, bodies of 


* C,H,+ H, = C,H,, 
Ch, Seeshee eh sc, 
CH, +C,H,—H,=C,H,, 

3CH, —2H, ay Be 7 Oo 


ISOMERISM. 289 


the same composition having a different origin may be 
identical ; they can only be regarded as isomers when 
it can be proved that they possess different properties. 

In the case before us, propane, whatever its mode of 
formation, is one and the same body, and, before admit- 
ting that it can give rise to particular isomeric deriva- 
tives by the mere fact of difference in the methods of 
its formation, it would be necessary to show that it 
receives from the latter, in each case, particular proper- 
ties, pointing to a difference of constitution. In a 
word, it would be necessary to prove that bodies formed 
by different reactions are isomeric. 

This may happen in certain cases; in the particular 
case which we are discussing it is not so, for there is 
nothing to show that propanes formed in different ways 
differ from each other in their structure, or, as Berthelot 
expresses it, in the residues which they contain. In 
trying to discern the residues of generating hydrocarbons 
in a complex hydrocarbon, Berthelot endeavours to 
determine the molecular structure of the latter. Fur- 
ther, in chlorine derivatives he marks the place of 
chlorine in each of these residues. He constructs con- 
stitutional formulz from the modes of formation of the 
hydrocarbons. Nothing is more legitimate in principle; 
but, unfortunately, the reactions which he takes as 
examples are not those from which any certain conclu- 
sion upon the constitution of bodies can be deduced : 
they are due to the action of heat, the most powerful 
form of reaction. 

Passing to another point, what is the meaning of 
this expression, ‘ generating hydrocarbons’? It is far 


290 THE ATOMIC THEORY. 


from expressing a definite idea. Also is it not a 
gratuitous supposition to admit that one chlorinated 
body can differ from another chlorinated body solely 
according to the order in which the substitution has 
been effected? Doubtless, when we substitute for several 
atoms of hydrogen different elements or groups—for 
example, chlorine and bromine, or chlorine and nitry] 
residues, NO,—the order of substitution is by no means 
an indifferent matter, because it is not a matter of 
indifference which place is occupied by chlorine or 
bromine, or by a nitryl residue ; but when all the places 
are occupied by chlorine, it matters not whether this or 
that place is occupied first. In any case the contrary 
supposition requires proof. Upon this subject Berthelot 
has accumulated hypotheses with an ingenious fecundity 
which in the present case has produced a remarkable 
result. The eminent chemist admits the possible exist- 
ence of several hundred trichlorhydrins from a conside- 
ration of the relative order of reactions.! We say that 
there are five. Where is the sixth? In the case of 
propane or propyl hydride we maintain that no isomers 
are predicted by theory. Between three atoms of carbon 
and eight atoms of hydrogen there is but one possible 
arrangement—two groups of CH, united to one group of 
CH,, as shown in the formula 


CH, 


| 
CH, 


CH, 


This formula is founded upon the consideration that 


> Bulletin de la Société chimique, nouv. sér., t. xiii. p. 402. 


ISOMERISM. 291 


the carbon atoms are quadrivalent, and that they can 
interchange a portion of their capacity of saturation. It 
is in this manner that the theory of atomicity predicts, 
interprets, and limits the number of isomers; it has 
furnished the elements of one of the greatest advances 
which science has accomplished in the last twenty years. 
It was generally said that isomerism is due to the 
difference in molecular grouping. This was stating the 
problem ; the next thing was to solve it. The theory 
of atomicity has successfully attacked the problem by 
introducing into the discussion exact data, which have 
been in a great number of cases confirmed by experi- 
ment. 


We 


This point is so important that I must ask 
permission to demonstrate by a fresh example the 
considerations by which chemists are now guided in the 
interpretation of isomers, and generally in conceptions 
relative to the constitution of bodies. 

Let us take a hydrocarbon which has been the 
subject of a great number of experiments, amylene. 

This body contains five atoms of carbon and ten 
atoms of hydrogen, two less than the saturated hydro- 
carbon of the series, amyl hydride or pentane, C,H... 
Starting from the idea of the quadrivalence of carbon, 
theory predicts at least eight possible arrangements 
of the five carbon atoms and the twelve atoms of 
hydrogen. In the first place, the latter may be equally 
divided among the former, so that each atom of carbon, 
riveted by two atomicities to its neighbours, is com- 


292 THE ATOMIC THEORY. 


bined with two atoms of hydrogen.’ There is every 
reason to believe that amylene thus constituted does 
not exist, and that this is also the case with those in 
which three or four carbon atoms present a similar 
arrangement. It should be remarked that bodies thus 
constituted would be in a manner saturated, all the units. 
of saturation belonging to the five carbon atoms being 
employed either in fixing hydrogen or in uniting the 
atoms of carbon. Now, all known amylenes? present the 


? On this hypothesis, which recalls Kekulé’s celebrated hypo- 
thesis upon benzene, the atoms of carbon would form a ring, or 
closed chain, and the constitution of amylene, which would be the 
true homologue of ethylene, would be expressed by the following 
formula :-— 


H, 
C 
om 
H,C OH, 
H,C—CH, 


We can conceive, further, the existence of two amylenes in which 
three or four carbon atoms would form a closed chain, two atoms, 
or a single atom, of carbon forming a kind of appendix, as may 
be observed in the hydrocarbon derivatives of benzene. The follow- 
ing formule would express the constitution of these amylenes :— 


H,C—CH, H,C—CH, 
we 
CH H,C—CH 
| 
CH, CH, 
| 
CH, 


The five amylenes in question present the following constitu- 
tion :— 


ISOMERISM. 293 


character of non-saturated compounds: they absorb 
bromine with energy, a property which is easily 
accounted for theoretically by supposing that two of the 
carbon atoms which they contain exchange not one 
unit of saturation, but two, as is shown by the for- . 
mule given in the note below, which express the 
constitution of five different amylenes. In these for- 
mule the double bonds between two contiguous car- 
bon atoms are represented by a double bond, a most 
convenient form of notation, as it enables us to 
show at once the state of saturation of each atom of 
carbon. 

When an amylene thus constituted fixes two atoms 
of chlorine and bromine, or one molecule of hydro- 


CH, CH,; ~ CH, CH, CH, 
| | 
| I 
CH, CH CH  CH,— CH 
| | II | 
CH CH CH CH, CH, 
| | 
CH, CH, 
Ethyl-allyl; boiling Flavitzky’s Boiling Amylene derived Ordinary 
point 40° (?). amylene ; point from active amylic amylene ; 
boiling point 38°-40°. alcohol (Le Bel) ; boiling 
pried boiling point 32°. point 36°. 
CH, 
We may regard them as substituted derivatives of ethylene, || ,in 


2 
which cne or several atoms of hydrogen are replaced by various 


alcohol radicals. This conception enables us to express their con- 
stitution by relatively simple formulz, which we give in the same 
order as above :— 


CH(C,H,) CH(C,H,)! CH(C,H,) C(CH,.C,H,) O(CH,), 
| 
CH, CH, CH(CH,) CH, CH(CH,) 


Propyl-ethylene. Isopropyl- Normalethyl- Methyl-ethyl- Trimethyl- 
ethylene. methyl-ethylene. ethylene. ethylene. 


294 THE ATOMIC THEORY. 


chloric acid, or a molecule of hypochlorous acid, the 
double bond is suppressed; the two contiguous carbon 
atoms, now united by a single atomicity or valency, 
each fix an atom of chlorine, or an atom of bromine, 
or an atom of hydrogen, or the group OH; and we may 
thus imagine isomeric compounds to be produced, 
according to the, place occupied by the atoms fixed 
in the molecule. Thus the isomerism of the amylene 
chlorides and bromides, and of the amyl chlorides, bro- 
mides, iodides, and hydrates, arises from the isomerism 
of the amylenes, and we see that all these isomers are 
accounted for by theory, which shows how they each 
correspond to a particular atomic grouping. Further, 
it is important to remember that the formulz which 
express this grouping are not chosen at random; they 
represent facts—that is to say, syntheses, reactions, and 
decompositions. Thus the formula of propyl-ethylene, 
(ethyl-allyl) recalls the synthesis of this body by the 
action of sodium upon a mixture of ethyl iodide and 


allyl iodide. ; 
CH,—CH,I + CH,L—CH=CH, + Na, 
Ethyl iodide. Allyl iodide. 


= 2NalI + CH,—CH,—CH,—CH = CH, 
Propyl-ethylene. 


The same formula accounts for the transformation 
of the hydrocarbon in question into an iodide and into 
secondary amyl alcohol. For greater simplicity we shall 
here employ the condensed formule— 


C,H, C,H, 
| : 

Gder Hit =) (CHT 
| 

CH, CH, 


Propyl-ethylene. Secondary amyl iodide. 


ISOMERISM. 295 


C,H, C,H, 
HI + AgOH! = AgI + CH—OH 


OH, CH, 
Secondary Secondary 
amyl iodide. amyl alcohol. 


And this secondary alcohol differs in its properties 
from two other secondary alcohols which were predicted 
by theory, and which have been obtained. Thus it 
differs from the tertiary alcohol, which is formed by 
the action of water and silver oxide upon amylene 
hydriodate or tertiary amyl iodide ; the latter is the 
principal product of the action of hydriodic acid upon 
ordinary amylene. To enable the reader to form an 
idea of the variety of isomeric compounds which may 
exist for a relatively simple grouping of atoms, such as 
that of amyl alcohol, we have given in a note? the 
table of all the known isomers of amyl alcohol. They 
were predicted by theory and verified by experiment ; 
and this verification of the theory, this happy coinci- 
dence between predicted and observed facts, may be 
noticed in hundreds of cases. Thus far we are justified 
in the assertion that the notion of atomicity has 
furnished sure data for the interpretation of isomers. 

We shall add but one more example to the preceding 
developments, which it would be easy, though super- 
fluous, to extend. We have spoken above of primary, se- 
condary, and tertiary amyl alcohols. This is an im- 
portant conception of Kolbe, and is demonstrated by the 
following formulz :— 


' Instead of Ag,O + H,O. 
2 See Note III., p. 336. 


296 THE ATOMIC THEORY. 


CH,—CH,—CH,--CH,—CH,.0H normal amylic alcohol, primary. 


CH,\ fermentation amyl alcohol, pri- 
cH,» CH—CH,—CH,.0H ae 


CH,—CH,—CH,—CH.OH—CH, { Sh carbinol, second- 
nO OH—CH,—CH, tertiary amyl alcohol. 


Those alcohols are termed primary which coatain in 
the chain of their carbon atoms the group —CH,.OH ; 
secondary, those which contain the group —CH.OH; 
tertiary, those which contain the group =C.OH. The 
primary alcohols alone give on oxidation aldehydes and 
acids, and the notation accounts for this important fact, 
for the group —CH,.OH alone can be converted by oxida- 
tion into the group —CHO or into the group —CO.OH.' 
Now the group —CHO characterises the ae and 
the group —CO.OH the acids. 

But what becomes of the secondary and tertiary 
alcohols when they are subjected to oxidation? One of 
two things, either their molecule is broken up or it is 
formed into a corresponding acetone. In the latter 
case the group —=CH.OH is converted into a group =CO, 
which characterises the acetones.? 

It is evident that the developed formule which 
express the reciprocal bonds between atoms, or, in other 
words, the atomic chain, account in the most satisfac- 
tory manner for the transformations which organic com- | 


1 CH,—CH,.OH + 0 = H,0 + CH,—CHO. 


Alcohol, Aldehyde. 
CH,—CH,.0H +0, = H,O + CH,—CO.OH. 
Alcohol. Acetic acid. 


2 CH,—CH.OH—CH, + 0 = H,O + CH,—CO—CH, 


Secondary isopropyl Acetone. 
alcohol, 


ISOMERISM. 297 


pounds may undergo. They show that a definite atomic 
grouping corresponds to each function, and that the 
fundamental properties of each class of bodies are in 
some way dependent upon this particular grouping. 
We can only give here a general statement of this pro- 
position; its proof would require a volume.! In our 
statement of this proposition, which we have supported 
by a few examples, we have had but one aim in view 
—to show that the developed formulz deduced from 
the theory of atomicity express facts, and that, when 
grounded upon the faithful interpretation of reactions, 
they are of the greatest use in the explanation of cases 
of isomerism. It is true that such formule are some- 
what complicated. This complication naturally in- 
creases with the number of atoms in the molecule 
whose atomic grouping we wish to represent. To this 
we must resign ourselves; the problems which science 
is called upon to solve are not always simple, and in 

1 Let us take one from a great number of examples. Acids, we 
say, contain one or several CO.OH groups. This enables us to ex- 
plain in avery simple manner the transformation by the action of 


heat of a great number of acids: the carbonic anhydride which is 
disengaged comes from the group CO.OH. 


C,H,—C0.0H =. CH + CO, 

Benzoic acid. Benzene. 

- CO.OH 

OHA See = C,H, £ 200, 
Benzene, 

OH 
CHK 6 eee = 0,H,—OH + CO, 
Salicylic acid. Phenol. 

C,H,(OH),—CO.OH = C,H,(OH); + CO, 

Gallic acid. Pyrogallol. 

0,(CO.0H), — 0,H, + 6CO, 


Mellic acid. Benzene. 


298 THE ATOMIC THEORY. 


the present case the difficulty is in reality small and 
only likely to repel amateurs of the science. The latter 
generally ignore the existence of amyl alcohols and iso- 
meric amylenes; their opinion, however, is of little im- 
portance, and it would be waste of time to discuss it. 


Mele 


Constitutional formule are based upon the principle 
of the reciprocal saturation of atoms or of the atomic 
chain, a principle which follows from the notion of 
atomicity, of which it 1s the most important conse- 
quence. It has its source in facts, and is of practical 
utility, for chemists are continually making use of it in 
discussions relative to the constitution of bodies and to 
the interpretation of their preperties. But it is neces- 
sary to make, or rather to renew, a limitation to the 
signification of these constitutional formule. Although 
they indicate the relations between atoms, they do not 
pretend to mark their position in space. The latter 
problem, which relates to the form of molecules, lies 
beyond the sphere of positive chemistry, although this 
science may furnish elements for its future solution. 
An attempt has, however, been made to solve it, so 
that we are justified in offering a few remarks on the 
subject. 

A molecule formed of two atoms, such as hydro- 
chloric acid, evidently possesses a simple form, which 
is linear if, instead of considering the atoms them- 


selves, we consider the mean positions of their centres’ 


of gravity. 


ATOMIC CONSTITUTION, 299 


Sodium chloride corresponds to hydrochloric acid, 
but the molecules of these bodies are only comparable 
when considered in the gaseous state. When solid or 
crystallised, sodium chloride is unquestionably formed of 
several molecules. Supposing that in a cube the eight 
angular points are occupied by atoms of chlorine and 
sodium, it would require four molecules of sodium chlo- 
ride to form an elementary cube of this substance. 
Similar considerations apply to other solid bodies, which 
may be formed of aggregations of molecules: dimorphism 
is explained by the diversity of these molecular aggre- 
gations. 

A molecule formed of three atoms, such as water, 
may be constituted in such a manner that one of the 
atoms—oxygen, for example—being placed at the 
centre of a circle, the two others occupy the extremity 
of a diameter. 

In ammonia we have four atoms, one of nitrogen 
and three of hydrogen, and it is probable that the 
latter are distributed symmetrically round the atom of 
nitrogen. Considered in their mean positions, all four 
may be arranged upon one plane, though it may be 
otherwise. 

The case of a molecule formed of five atoms, such 
as marsh gas or methane, is more complex. The most 
plausible hypothesis consists in again admitting that 
the four hydrogen atoms are symmetrically distributed 
round the carbon atom. The latter may be imagined 
to be placed in the centre of a regular tetrahedron, of 
which the four hydrogen atoms would occupy the four 
angular points. 


300 THE ATOMIC THEORY. 


A few more words upon this hypothesis. It is sus- 
ceptible of an important development, which we owe to 
Le Bel and Van’t Hoff, and which we now proceed to 
describe. Speaking correctly, we shall only consider a 
particular case of a more general proposition enunciated 
by these chemists; but this simplified demonstration 
will suffice to give an idea of their conception. 

A great number of organic compounds may be 
derived from marsh gas, regarded as a type of com- 
bination. 

In fact, the hydrogen atoms may be replaced by 
other elements, or groups of elements, and especially 
by carbon groups. An immense variety of combina- 
tions may result from these substitutions, and the mole- 
cule grows from the complication of the groups in 
question. But the primitive carbon atom will constitute 
a kind of nucleus.! Suppose that for the four atoms 
of hydrogen we substitute four identical groups—four 
ethyl groups, for example—the form of the molecule 
will be symmetrical, as it was before; and if this form 
is a regular tetrahedron, a plane passing by an edge 
and through the centre of the figure occupied by the 
carbon atom would divide the molecule into two sym- 
metrical halves. This plane of symmetry would com- 
prise two angular points, the two others being situated 
upon a line perpendicular to this plane and at an equal 
distance from it. 

Let us now suppose that of the four hydrogen atoms 
of marsh gas two are replaced by groups of atoms, and 


1 This does not necessarily imply a peculiar condition of the 
carbon atom. 


*. 


—~—- 


MOLECULAR DISSYMMETRY. 301 


that the latter are situated upon the plane in question ; 
the two other hydrogen atoms will be situated at equal 
distances upon a line at right angles to this plane, and, 
were the plane a mirror, the image of one of the 
hydrogen atoms would coincide with that of the other. 
This would no longer be the case if the substitution 
were extended to a third hydrogen atom, in such a 
manner that the molecule would now contain three 
dissimilar elements or groups, R, R’, R”’; in this case 
the elements upon each side of the plane passing 
through H and R will be R’ and R”, and those situated 
upon each side of the plane passing through R’ and 
R” will be H and R; they are dissimilar in the two 
cases, and the structure of the molecule will be dis- 
symmetrical. This molecular dissymmetry, thus de- 
fined by Le Bel and Van’t Hoff, is the cause of rotatory 
power. 

Let us take as examples propionic, lactic, and 
tartaric acids. | 

Propionic acid is a bisubstituted derivative of 
marsh gas. 


CH, 
| 
H—C—H H—C—H 
H CO—OH 
Marsh gas. Propionic acid. 


Let us suppose the molecule to be divided by a 
plane in such a manner that CH,, C, and CO.OH shall 
be situated upon this plane; the two halves will be 
symmetrical, an atom of hydrogen being situated right 
and left. Propionic acid does not exercise a rotatory 
power. 


302 THE ATOMIC THEORY. 


Lactic acid is a trisubstituted derivative of marsh 
eas of the form 
CH, 
aay te 


CO—OH 
Lactic acid. 
In this case the plane passing through CH,, C, and 
CO—OH will no longer be a plane of symmetry, for 
of the two symmetrical hydrogen atoms which existed 
in propionic acid one has been replaced by OH; the 
molecule will therefore be dissymmetrical. Lactic acid 
exercises a rotatory power. | 
This is also true for tartaric acid, which may equally 
be regarded as a trisubstituted derivative of marsh gas, 
assuming the general form 


R 
H—C—R’ 


R". 
Tartaric acid being 


we see that one of the carbon atoms is united to a hy- 

drogen atom, and to three different groups, R” being 
CH.OH 

in this case | . From what has been said above, 


CO.OH ~ 


the structure of the molecule is dissymmetrical. 


MOLECULAR DISSYMMETRY. 303 


The objection will, however, be made that the lactic 
acid of fermentation is inactive, as are also many other 
bodies presenting a similar structure, Le Bel sets this 
difficulty aside by observing that, in a bisubstituted 
derivative of marsh gas, the third substitution may take 
place on either side, to left or to right, and that the 
dissymmetrical compounds thus formed are in reality a 
mixture in equal proportions of dextro-rotatory and 
levo-rotatory bodies, and are consequently inactive. 

We may add that the above course of reasoning 
implies an hypothesis—that, namely, of the fixity of 
hydrogen atoms and groups, relatively to each other, in 
the types CHR,R’ and CHRR’R”. If this were not so, 
if the hydrogen atoms and the groups which replace 
them could be continually changing places in this system 
and alternately occupy all positions, the molecular dis- 
symmetry, as defined above, could produce no effect, 
because in the mass of the molecules we should continu- - 
ally find dissymmetrically opposed compounds in equal 
proportions, and consequently optical neutrality. On 
the other hand, the grand fact of isomerism demonstrates 
the stability of the relative positions of atoms and groups 
for an immense number of combinations. It is well 
known that to determine, in a given combination, the 
migration of atoms or groups which produces isomerism, 
the intervention of a physical force or a chemical agent 
is necessary—for example, a great elevation of tempe- 
rature. How could the innumerable isomers of the 
innumerable derivatives of benzene occur, if the six hy- 
drogen atoms of benzene were not all riveted to their 
carbon atom, executing in its vicinity, without ever 


304 THE ATOMIC THEORY. 


quitting it, as long as the compound exists, those mo- 
tions which constitute a part of the total energy of the 
molecule ? 

We shall confine ourselves to these brief observations, 
which will at least have shown the cause of the interest 
which attaches to the attempt of Le Bel and Van’t 
Hoff. 

Biot often insisted upon molecular dissymmetry as the 
cause of rotatory power, and upon the assistance which 
this remarkable physical property of certain bodies 
would one day offer to the study of their constitution. 
This was a prediction which was fulfilled when Pasteur 
showed ‘the relation which exists between rotatory power 
and crystalline dissymmetry. But with liquid or dis- 
solved substances it is no longer a question of crystals, 
but of molecules, and the considerations relative to 
molecular dissymmetry were, as it were, the offspring of 
pure chemistry. They have been deduced from the 
theory of atomicity, and are connected with modern ideas 
upon the links existing between atoms in combinations. 


CHAPTER IV. 


HYPOTHESES UPON THE CONSTITUTION OF MATTER. 


E 


WE have now come to the end of this long exposition, 
and must conclude. 

We have pointed out the origins and followed up 
the development of this celebrated theory of atoms, 
which, from the first ages of civilisation, has been 
present to the human mind, seeking both to penetrate 
into the inmost recesses of matter and to sound the 
infinite depths of space. And we have a sound reason 
for comparing the ‘little world’ in which atoms are 
rotating to the great world in which the stars revolve. 
In both all is motion. We must go back to the very 
origin of atomism to find this conception of atoms in 
motion. It is mental power (vods) which gives them 
the impulse, according to Anaxagoras. According to 
Democrites of Abdera, they are in perpetual motion 
from their very nature ; the force which animates them 
acts inevitably. They do not differ in their essence— 
we should now say,in their chemical qualities—but 
rather in their dimensions, for they have a sensible 
extension; they differ also in their form. If heavy, 
they fall into the depths of space; if lighter, they rise 

14 


306 THE ATOMIC THEORY. 


in the air. Some havea smooth surface; others present 
asperities, points,and hooks. ‘The motion which they 
possess naturally brings them into contact, without 
their attracting each other; sometimes it masses them 
together, sometimes separates them. It is in this 
manner that all things are formed or destroyed. 
Limited in extent and surface, they cannot be con- 
founded with the medium in which they move. This 
medium is a vacuum. 

Thus we find, at the very origin of atomic ideas, 
this distinction between ‘vacuum and plenum,’ or 
‘vacuity and solidity,’ which was maintained for ages, 
and which appears as one of the solutions which the 
human mind has been able to give of the constitution 
of matter. This is the hypothesis of the discontinuity 
of matter, now generally adopted, with the difference 
that the vacuum is replaced by a very rarefied, elastic, 
vibrating medium—ether. 

Another hypothesis is that of continuous matter 
filling all space, with different degrees of density. 
Descartes inclined towards the latter hypothesis. Di- 
mension being the essential property of bodies, there 
cannot be bodies without dimensions, which excludes 
the idea of indivisible particles—that is to say, of atoms. 
There is no dimension without body, consequently no 
vacuum. 

The same conception follows from the dynamic 
hypothesis professed by German philosophers at the 
commencement of this century. Whether, after Kant, 
matter exists by itself, or is endowed with two con- 


trary forces, one attractive and the other repulsive, or ~ 


CONSTITUTION OF MATTER. 207 


results solely from the conflict of these two forces, as 
Schelling supposed, it is continuous, and consequently 
infinitely divisible. Chemical combination results from 
the mixture of heterogeneous bodies, which penetrate 
each other, and this penetration is so intimate that we 
cannot find in the compound either the properties or 
even the substance of the components ; a new substance 
is formed, and the smallest particle of it is entirely 
identical with the whole mass. 

We are now struck with the vagueness of the dynami- 
cal ideas of Schelling. No force can exist independently ; 
it must emanate from something or must be applied 
to something which exists apart from it, and should 
manifest its presence by motion. How can we con- 
ceive motion without a moving body? For this no- 
tion of force, which is, moreover, difficult to define, 
we may substitute that of motion. In the hypothesis 
of the continuity of matter the mass which fills the 
entire universe is in a permanent state of vibratory 
motion, Waves are transmitted through it in different 
directions and cross each other, as the waves produced 
on the surface of water are transmitted and cross with 
other waves. rom the intersection of the waves result 
nodal surfaces and nodal points, and consequently the 
limited portions of matter. The diversity of matter 
would result from the diversity of the systems of waves 
which traverse it, and we can logically conceive that 
these portions thus limited, these vibrating slices, these 
concamerations, if we may use the word, represent the 
particles of matter which enter into conflict in chemical 
reactions. 


308 _ THE ATOMIC THEORY. 


This is an hypothesis, and it seems vague and to 
possess but slight plausibility, at least under this form. 
It is otherwise with a conception of Sir William 
Thomson, who has recently given a striking definition 
of these limited portions of matter vibrating in the 
midst of a continuous medium. This definition we shall 
explain further on. 


It: 


We will, however, consider more closely the hy- 
pothesis of the discontinuity of matter which would be 
formed of molecules and atoms in motion, in a medium 
which fills the entire universe and penetrates all 
bodies—the ether. 

Atoms are not material points; they possess a 
sensible dimension, and doubtless a fixed form; they 
differ in their relative weights and in the, motions 
with which they are animated. They are indestructible 
and indivisible by physical and chemical forces, for 
which they act, in some manner, as points of appli- 
cation. The diversity of matter results from primordial 
differences, perpetually existing in the very essence of 
these atoms and in the qualities which are the mani- 
festation of them. 

Atoms attract each other, and this atomic attraction 
is affinity. It is doubtless a form of universal attraction, 
but it differs from it in that, if it is obedient to the 
influence of mass, it depends also on the quality of the 
atoms. Affinity is elective, as has been said for a 
hundred years. It gives rise to aggregations of atoms, 


ad 


CONSTITUTION OF MATTER. 309 


to molecules and chemical combinations. In the 
latter the atoms are no longer free in their mo- 
tions; they execute their motions in a kind of co- 
ordinated manner, and constitute a system in which 
everything is solid and in which they are under 
control. This is a molecule. It has a determinate 
mass, a centre of gravity, and its peculiar motion. 
The energy of these molecular motions determines a 
very important physical condition—temperature. We 
shall return to this point later. 

Ether is not a vacuum; it isa medium formed of 
a highly rarefied and elastic matter, agitated by per- 
petual vibrations which are transmitted from atomic 
matter to the ether, and from the ether to atomic 
matter. Is it a homogeneous, continuous medium ? is 
it formed of atoms of a second order, a kind of monads, 
which would form by their ageregation ponderable 
matter itself? This is a question which can be 
brought forward, but which cannot be solved. Poisson 
inclined towards the hypothesis of the discontinuity of 
the ether; it seemed to him difficult to admit that 
luminous vibrations could be propagated transversely 
in a continuous medium. Such as it is, this im- 
perceptible and imponderable medium has, never- 
theless, a density, and, after a revived hypothesis of 
Lesage—that of transmundane atoms—it is the influence 
of this fluid which would cause the stars of the visible 
world to gravitate towards each other. 

The medium, therefore, extends between all parts of 
the universe. As a transmitter of radiations it receives 
and transmits to us, under the form of light and heat, 


310 THE ATOMIC THEORY. 


the radiations—that is to say, the vibrations— impressed 
“upon it by the sun and the most distant stars, and 
sends into space those which proceed from our solar 
world. And the same exchange is established in the 
infinite domain of the infinitely small. The atoms and 
the molecules which move with different velocities in 
this impressionable medium communicate to it a part 
of their energy, which is propagated through it under 
the form of radiant heat and light; and, reciprocally, 
the calorific and luminous waves of the ether which 
graze the surface of atoms or groups of atoms augment 
the amplitude of their trajectory and the energy of 
their vibratory motions. It is this incessant com- 
munication of motions, this perpetual exchange of 
energy between the ether and atomic matter, which 
gives rise to the most important phenomena of physics 
and chemistry. | 

Among such a varied class of phenomena the 
hypothesis of atoms establishes relations which no 
other theory has foreseen, and which are so simple and 
so well known that it is almost superfluous to mention 
them. 

Take a crystal. Under the microscope its mass 
appears compact and homogeneous. In its faces and 
cleavage planes there is no absence of continuity. And, 
nevertheless, matter is not continuous in it, and, if the 
substance is a compound, it is not homogeneous. The 
smallest fragments of the crystal are formed of aggre- 
gations of innumerable molecules, which are similar in 
nature and similarly arranged. Each of these molecules 
is formed of atoms, varying in number. They are 


CONSTITUTION OF MATTER. 311 


situated at distances sensible compared with their 
dimensions, and they vibrate in a coordinated manner, 
forming systems in equilibrium, each of which is 
animated with determinate motions and is in accord 
with systems of the same kind. If the body in ques- 
tion is a solid, the atomic systems—that is to say, the 
molecules which constitute it—preserve their respective 
positions, and are linked together and fixed with regard 
to each other, although each one has its own orbit and 
a certain liberty of motion. It is cohesion, we say, 
which retains the molecules in their spheres, and 
affinity which retains the atoms in the narrower limits 
of the molecule. But these forces are perhaps 
fundamentally of the same nature: only they act at 
different distances, and, under the influence of the same 
causes, they are manifested differently, one giving rise 
to physical phenomena, the other to chemical pheno- 
mena, the latter only being in some manner the con- 
tinuation of the former. 

If we.apply heat to a solid body! formed of mole- 
cules thus constituted, it can produce, independently 
of external work, three different effects— 

Firstly, an elevation of temperature, owing to the 
increase of the molecular vibratory energy ; 

Secondly, augmentation of volume from the increase 
in the space between atoms and molecules, and, if this 
increase becomes considerable, a change of state: the 
solid becomes a liquid, the liquid a gas. In the latter 
case the separation of the molecules has become con- 
siderable compared with the dimensions of the latter 

1 See note cn p. 338. 


312 THE ATOMIC THEORY. 


But, whatever may be the physical effect produced in 
this order of phenomena, the heat which has disappeared 
as such has performed work; the vibratory motion 
which has been communicated to the molecules under 
the form of heat has succumbed in the struggle against 
the molecular forces, or, in other words, has performed 
work represented by dilatation, diminution of cohesion, 
and change of state. 

The phenomena which we have just analysed are 
physical ones. Chemistry also comes in play; for, in 
the third place, heat, acting on the atoms themselves 
which make up the molecule, increases their trajectories, 
so that the equilibrium which existed in the system is 
destroyed, and the atoms of one given system pass into 
the sphere of action of the atoms of another system. 
From this rupture or this conflict new systems of equi- 
librium result—that is to say, new molecules. This 
gives rise to phenomena of dissociation or of decom- 
position, and inversely of combination, which belong 
to the domain of chemistry; as is seen, and as we have 
mentioned above, they are only the continuation of 
the. physical phenomena, the same hypothesis—that of 
molecules formed of atoms—applying to each with equal 
simplicity. 

It is heat which sets the atoms in motion; they 
have absorbed heat in separating from each other, since 
the rupture of the molecular equilibrium which marks 
the end of the state of combination has required the 
consumption of a certain quantity of heat. The heat 
thus absorbed has restored to the atoms the energy 
which they possessed before combination and which 


i rl 


CONSTITUTION OF MATTER. 313 


represents affinity. This heat is lost again whenever 
the atoms, passing into the sphere of action of other 
atoms, fix the latter in some manner or are fixed by 
them, so as to form new systems of equilibrium—that 
is, new molecules—in which henceforth their vibration 
and motion are preserved. This action is reciprocal ; 
the new combination can only be formed on the condi- 
tion that the motions of the atoms which constitute 
it adapt themselves in some manner to each other and 
harmonise, losing thereby part of their vibratory and 
potential energy. Hence the disengagement of heat. 
It is also evident that this adaptation should require 
certain conditions of modality. All kinds of motion 
cannot agree in the same manner, and the harmony of 
the molecular motions should be influenced by the 
manner of the atomic motions. This circumstance, 
added to the differences inherent in the very nature of 
atoms, determines the variety of the systems of equi- 
librium, or, in other words, the different forms of com- 
bination. There intervenes a peculiar property of 
atoms, very different from their chemical energy. In 
order to distinguish it from affinity we have called this 
property of atoms atomicity, and we suppose that it is 
connected with their very nature and kind of mo- 
tions. 

But are not all these vibratory and rotatory mo- 
tions which continually agitate molecules and atoms, 
to which may be added the sliding motions of 
liquids and the rectilineal progression of gases, so 
many causes of instability to the molecular systems ? 
The case is the contrary. If they are immovable the 


314 THE ATOMIC THEORY. 


atomic aggregations will be more unstable than when 
they are in motion; the familiar example of the 
bicycle shows the influence of motion upon the stability 
of equilibrium. 


IMO i 


We have mentioned above the differences inherent 
in the very nature of atoms and in their manner of 
motion as determining the form and nature of chemical 
compounds. The molecular movements have lately 
been the object of important researches, principally 
upon the physical constitution of gases, which have 
resulted in unexpected revelations not only upon the 
movements and velocities, but even upon the reciprocal 
distances and absolute dimensions of the ultimate par- 
ticles. Is this an illusion, a scientific fantasy? No; it 
is a serious undertaking, a powerful effort made by the 


most eminent physicists and geometricians, and appears. 


to us worthy of attention, though the future may 
demonstrate its insufficiency. We will close this ex- 
position with some words upon the subject, warning 
the reader that we are here entering the invisible 
world, unassailable by direct experiment, but add that 
the hypotheses which give us access to it can be 
verified in some of their consequences and hence acquire 
a certain degree of probability. 

Daniel Bernouilli was the first to bring forward the 
idea that gases are formed of small material particles, 
animated with very rapid rectilineal motion, and that 
the tension of elastic fluids resulted from the impact of 


CONSTITUTION OF MATTER, 315 


their molecules against the sides of the vessels in which 
they were contained. This is the origin of the kinetic 
theory of gases, which has been taken up by Herapath, 
Joule, and Kronig, and the principal author of which is 
Clausius. Clerk Maxwell has added very important 
developments to the theory. 

Mariotte’s law follows as a natural consequence of 
this hypothesis upon the constitution of gases. If a 
gaseous mass, composed of an immense but determinate 
number of particles, is contained in a closed space—the 
cylinder of an air pump, for instance—the pressure 
exerted upon the piston will be determinated by the 
normal components of the impact of the molecules dur- 
ing aunitof time. If the volume of this gas is reduced 
the number of particles will increase, as well as the 
number of impacts: the pressure will, therefore, undergo 
a proportionate increase. 

The volume remaining constant, the pressure will 
be also increased by an elevation of the temperature. 
Under these conditions the number of the particles 
remains the same; their masses remain constant, but 
their velocities, and consequently the number of impacts, 
is increased, and it is precisely the vis viva or the total 
energy of the molecular motions which give a measure 
of the temperature. 

After the preceding it is evident that a relation 
exists between the pressure or the tension of a gas and, 
on the other hand, the velocities of its molecules, their 
individual mass, and their number in the unit of volume. 
For these latter factors we may substitute the notion of 
density, which is the mass of the unit of volume. We see, 


316 THE ATOMIC THEORY. 


therefore, that the absolute velocities of the molecules 
can be calculated from the pressure and the density. 

These calculations have been made by Clausius. 
According to him the molecules of air move with a 
mean velocity of 485 metres per second, and the mole- 
cules of hydrogen with a mean velocity of 1,844 metres. 
A body projected vertically upwards with a velocity of 
485 metres per second would reach a height of 12,000 
metres before falling. 

Can the gaseous molecules which move with such 
velocity pass freely over such enormous distances? No; 
their number is so immense that at every instant they 
come into contact, encounter each other, and rebound, 
so that in a gaseous mass formed of molecules of the 
same kind the latter move in every direction with vari- 
able velocities and, between two encounters, in sensibly 
rectilinear directions. What happens when a gas thus 
constituted is mixed with another on which it has no 
chemical action? The molecules of the two fluids 
begin to diffuse into the space opened to them; pre- 
serving their velocities, they encounter each other, so 
that the sum of their impacts represents the total pres- 
sure excited by the mixture. In other words, the 
pressures of the two gases are added together. This is 
Dalton’s law, which also follows as a natural conse- 
quence from the hypothesis of Daniel Bernouilli. 

The gases just considered are perfect gases. Their 
molecules, freed from all cohesion, exert upon each 
other neither attraction nor repulsion, and, if they 
encounter each other, it is chance which brings them 
tovether. But in reality perfect gases do not exist. 


ae 


~~ |! ee a eee Pe 


CONSTITUTION OF MATTER. St 


When gaseous molecules approach to within very small 
distances of each other, cohesion begins to exert a 
sensible influence, which is transitory, since it decreases 
very rapidly with the distance. This is the cause of 
the departures from Mariotte’s law and of the inaccu- 
racies of Dalton’s law. It is also the cause of slight 
flexions in the molecular paths, when the particles are 
on the point of touching. 

The preceding considerations are of a_ physical 
nature and apply to molecules. The latter may be 
formed of several atoms, retained by affinity in their 
reciprocal spheres of action: these atoms have their 
own peculiar motions, their own energy in the system 
of which they form a part, and are constrained to move 
with it in the molecular paths. 

The mean molecular velocities! are different for 
different gases at the same temperature; the rectilineal 
paths passed over between two successive paths are also 
different. ‘The authors of the kinetic theory of gases— 
Clausius and Clerk Maxwell—have attempted to calcu- 
late the mean lengths of these paths and the number of 
impacts during a unit of time. These numbers can be 
determined in absolute value, if certain factors given by 
experiment are introduced into the calculation. One 
of these factors is the coefficient of friction of gases. 

A gaseous mass which is in motion upon the sur- 
face of a solid body tends to transmit, by a kind of 

1 We shall consider here the mean velocities, and add that ina 
homogeneous gaseous mass—that is to say, in a mass formed of the 
same kind of molecules—all do not move with the same velocities; - 


in reality, there are some which are endowed with very different 
velocities. This has been established by Clerk Maxwell. 


318 j . THE ATOMIC THEORY. 


friction, a portion of its motion to this body. This 
causes a retardation of the motion of the gaseous par- 
ticles in the layer which is immediately in contact 
with the solid body: this has been termed the external 
friction. But the layer in question undergoes in its 
turn a kind of impulse from the neighbouring layer, 
which is moving faster and which transmits to it a 
portion of its motion as a mass. Retardation, there- 
fore, takes place on one side and acceleration upon the 
other: this has been termed internal friction. But this 
transmission of motion cannot take place without loss, 
a portion of the motion of the mass being converted into 
calorific motion. All friction causes a disengagement 
of heat, and the calorific motion is characterised by the 
molecules moving in every direction, while they are 
impelled in the same direction in a gaseous current. 
The internal friction which the different layers of a gas 
in motion exercise upon each other gives rise, therefore, 
to a disengagement of heat—that is to say, to an accelera- 
tion of molecular motion. This transmission of motion 
-can only be produced by the impact of the molecules, 
and we see that a relation exists between the internal 
friction of gases on one side and the number and energy 
of the impacts during the unit of time on the other. 
The number of the impacts itself depends upon the 
velocity and the distances passed over by the molecules 
between two impacts. In a word, the coefficient of 
friction of gases, which has been determined by experi- 
ment, can be expressed in terms of the density of the gaa, 
of the molecular velocity, and of the distances passed over 
between two impacts—that is to say, of the molecular 


ee 


CONSTITUTION OF MATTER, 319 


paths. The latter can, therefore, be calculated. With- 
out giving here the numbers obtained for the ditferent 
gases, we will only mention that the distance passed over 
by a molecule of air, at the temperature of 0° and under 
the normal pressure, has been determined—an average 
and approximate value, since air is a mixture—as equal 
to ninety-five millionths of a millimetre, which is about 
twenty-five times smaller than the smallest magnitude 
visible under the microscope. The number of impacts 
received by such a molecule during one second will be 
4,700 millions, supposing it to move with a mean 
velocity of 447 metres. 

We may go further. The knowledge of the velocities 
which animate the molecules and the number of impacts 
has made it possible to draw inductions upon their 
distances, their diameters, and their volumes, for is it 
not true that the lengths of the molecular paths depend 
upon the frequency of the impacts, and that the latter 
is influenced by the size of the projectiles? We will 
not here quote numbers, and will only point out the 
course of ideas. 

Other considerations have been made use of for the 
approximate determinations of molecular diameters—in 
the first place, the density of a gas and that of the liquid 
resulting from its condensation. Loschmidt, Sir William 
Thomson, and Clerk Maxwell have been engaged upon 
this question. We will give a word of explanation upon 
this subject. 

If we could suppose that gaseous molecules were 
brought into contact by liquefaction, the relation exist- 
ing between the real volume occupied by the material 


320 - THE ATOMIC THEORY. 


molecules in the unit of volume of a gas and this volume 
would evidently be given “by the relation between the 
densities in the gaseous and liquid states. This is the 
relation which Loschmidt terms the coefficient of con- 
densation. Now, the molecular diameter! can be ex- 
pressed in terms of this coefficient and of the mean 
length of the molecular paths—that is to say, in terms 
of two known quantities. It is certain that the values 
calculated in this manner would be too high and rather 
represent a superior limit, for the supposition that 
liquefaction brings the molecules in contact is evidently 
an inexact one. These values are, therefore, only approxi- 
mate. It is nevertheless a remarkable circumstance 
that the numerical results obtained in this manner have 
been confirmed in a satisfactory manner by means of a 
very different method which has been recently employed 
by Van der Waals. 

This physicist has attempted to determine the mo- 
lecular magnitudes, taking as his starting-point the 
departures from Mariotte’s law. We have said above 
that the cohesion of gases is one of the causes of these 


1 We are here treating not exactly of the diameter of the material 
molecule, but of the radius of what Clausius calls the molecular 
sphere. The molecular sphere is the portion of space which be- 
longs to the molecule, and within which no other molecule can 
penetrate. Py 

These- molecular spheres of action would occupy, according to 
Clausius, a volume eight times as great as the molecules themselves. 
Let us quote some numbers torender these ideas intelligible. Clausius 
admits that in a cubic centimetre of air the material molecules only 
occupy a third of a cubic millimetre—that is to say, the 3000th part 
of the total space—and that the molecular spheres of action occupy 
about the 400th part of it. 


CONSTITUTION OF MATTER. 321 


departures; the material extent of the molecules - is 
another, for it is evident that the space in which the 
molecules can execute their evolutions is not the same 
in reality as that occupied by the gas itself; it differs 
from the latter by the whole extent of the molecular 
volumes. It is conceivable that the experiments of V. 
Regnault upon the compressibility of gas might furnish 
data for the calculation of a constant representing the 
material extent of molecules, one of the causes of the 
departures from Mariotte’s law. The same constant has 
been calculated by Van der Waals from the variations 
of the coefficient of dilatation, and the values obtained 
have been a little lower in this case. The values in ques- 
tion deal with millionths or with fractions of millionths 
of a millimetre. This kind of magnitude would express 
the diameters of molecules and also their respective 
distances. Let us remark that this scale is nothing 
extraordinary for physicists. Are not the lengths of 
light-waves expressed in hundreths of millionths of a 
millimetre ? And in this connection we should remember 
that Cauchy has some time ago drawn attention to the 
fact that the distance between molecules in a refracting 
medium should be in relation with the wave-lengths. 
He has shown that dispersion—that is to say, the de- 
composition— of white light into the different colours of 
the spectrum could not take place if the material par- 
ticles of a refracting body were separated by distances 
infinitely small in relation to the wave-lengths. These 
distances and these lengths should be magnitudes of 
the same order. 

But let us return to gases. The data obtained for 


322 THE ATOMIC THEORY. 


the diameters of molecules have made possible the 
approximate calculation of their sections and of their 
volumes. [or air the two values are only small frac- 
tions, the first of a square, the second of a cube, whose 
side would be the millionth part of a millimetre. 

We can go a step further: the molecular volumes 
being known, we can calculate the number of molecules 
in the unit of volume of gases, and also their respective 
distances and their absolute weight. We here arrive 
at numerical results which confound the imagination, 
and the real signification of which we have some diffi- 
culty in seizing. A cubic centimetre of air would 
contain twenty-one trillions of molecules, a number 
which represents twenty-one times the cube of a 
million.! In accordance with the law of Avogadro and 
Ainpére, this number would be the same for the other 
gases. Suppose gas to be so rarefied that its pressure 
was reduced to a thousandth part of an atmosphere: the 
number of molecules contained in it would still be 
prodigious, although it would only be the thousandth 
part of the preceding. Only in this rarefied air the 
molecular paths would be notably elongated, and the 
number of impacts diminished in proportion. This 
furnishes an explanation of the possibility of the 
motion of the radiometer. 

The atomic weights, which we have treated at length 
in this work, only express ponderal relations. These 
weights can be expressed in absolute value by taking 


1 That is to say, 21 followed by eighteen cyphers. Since the 
words billion and trillion do not always receive the same meaning, 
I have thought this explanation necessary. 


CONSTITUTION OF MATTER, 323 


as the bases of the calculation the data acquired upon the 
dimensions of molecules, and taking into account the den- 
sities. If we say that it requires ten trillions of molecules 
of air and 144 trillions of molecules of hydrogen to make 
a milligramme of these respective gases, shall we be giving 
any idea which the mind can grasp, and, above all, will 
the idea of the values be exact? This may be doubted. 
Nevertheless these numbers have been given. They 
are given with reserve, as provisional information upon 
the limits which the divisibility of matter can reach. 
Besides, these approximate results upon the dimen- 
sion of the material particles and upon the density of 
this molecular dust which constitutes gases are con- 
firmed, up to a certain point, by well-known facts. An 
inappreciably small weight of musk is sufficient to per- 
fume the air of a whole room, and Kirchhoff and 
Bunsen have proved that the three-millionth part of a 
milligramme of sodium chloride is sufficient to give a 
yellow colour to a gas jet; and, in another kind of 
phenomena, Hcfmann has found that rosaniline gives a 
perceptible colour to 100 million times its weight of 
aleohol. We should also remember that Faraday pre- 
pared sheets of gold the thickness of which he estimated 
as equal to the hundredth part of the length of a light- 
wave; and, supposing that such a sheet contains a simple 
layer of molecules, the thickness of one of these could 
not exceed five millionths of a millimetre, a value 
comparable to those mentioned above. Everyone is 
familiar with a soap bubble, but it is difficult to form 
an idea of the extreme tenuity of the film. In the . 
soap films made by Plateau they scarcely reach a 


324 THE ATOMIC THEORY. 


millionth of a millimetre. Sir William Thomson, who 
has studied this question, has shown by calculation that 
it is impossible for such a film to contain more than one 
layer of molecules, which would give for the diameter of 
these molecules a value inferior to that just mentioned, 
but which belongs to the same kind of values. Finally, 
a last and very ingenious method for estimating the 
limit of the divisibility of matter has been conceived by 
Sir William Thomson. It consists in determining, by 
means of his electrometer, the voltaic attraction between 
a sheet of zinc and a superposed sheet of copper. This 
attraction is independent of the thickness of the sheets, 
and can be estimated by the work performed by the sheet 
of zine in lifting itself towards the sheet of copper. If 
thin layers of zinc and copper were made into a pile, 
this work would increase in consequence of the number of 
sheets accumulated in a given thickness. This mechani- 
cal work can be calculated in heat, and we can calcu- 
late the thickness which it would be necessary to give 
to the sheets for the work performed by a pile of sheets 
one centimetre in thickness to represent exactly the 
quantity of heat disengaged by zine and copper in 
forming an alloy. In other words, we can calculate the 
tenuity which it would be necessary to give to the par- 
ticles of zinc and copper in order that their reciprocal 
action should only disengage the quantity of heat which 
is actually observed in the formation of an alloy. The 
result is as follows: The thickness of these sheets cannot 
be reduced beyond the thirtieth of the millionth part 
of a millimetre. It is approximate, for there are 
uncertainties in the calculation, and it is an inferior 


CONSTITUTION OF MATTER. 325 


limit; but it is noteworthy that we still remain in 
the order of values deduced from the kinetic theory of 
gases. 


bye 


This theory, and especially the law of Clerk Maxwell 
upon the distribution of velocities in gaseous molecules, 
are founded upon the mathematical laws of the im- 
pact of elastic bodies. Are molecules, then, projectiles 
endowed with elasticity? The admission would be 
difficult if we wished to maintain the conception, which 
Newton arrived at, of hard atoms, incapable of altering 
their form. The difficulty is not diminished if we 
retain the hypothesis of molecules formed of atoms held 
together by affinity; and these are exactly the mole- 
cules which we have considered in gases as moving in 
every direction and encountering each other. Are they 
endowed with elasticity by reason of the extension of 
their sphere of action beyond their natural limits, 
according to the ideas of Clausius ? Are they surrounded 
with an envelope of ether? So it has been said, but 
who can affirm anything in the matter? And then there 
is afinal difficulty. This invisible world, in which it 
has been attempted to penetrate by an effort which isan 
honour to the human intelligence, has finite dimen- 
sions. It does not exist in absolute quantity, and in 
this unheard of diminutiveness there are relative 
magnitudes. Chemistry teaches us that a molecule of 
mercury weighs 100 times as much as a molecule of | 
hydrogen. Is it, therefore, a great molecule relatively 


326 THE ATOMIC THEORY. 


to the other, and why is it indivisible? I do not 
understand; I do not pretend to do so; only I admit 
that physical and chemical forces cannot divide it any 
further, because otherwise it would cease to be mercury. 
It is not less true that this proposition of the indivisi- 
bility of atoms is not to my mind satisfactory, and I 
am obliged to confess that a difficulty exists here. 

In these later times a theory has arisen which seems 
to give a mathematical demonstration, and even an 
experimental illustration, of the indivisibility, or rather of 
the peculiar and eternal individuality, of atoms: I refer 
to the vortex atoms of Sir William Thomson. 

Chemists can form an idea of this vortex motion by 
recalling to mind the rings which rise in still air 
whenever a bubble of phosphoretted hydrogen byrsts 
upon the surface of water, and the rings which certain 
smokers are able to make are familiar to all. An appara- 
tus has been constructed by which they may be produced 
at will. It is a wooden box one side of which is fur- 
nished witha circular opening and the other formed of a 
tightly stretched cloth. In the interior of the box 
fumes of sal ammoniac are produced, which are driven 
out by a sharp blow on the elastic side. A ring of 
smoke is then seen to issue from the opening and to 
move freely through the room. In this ring all is 
motion, and, independently of the motion of translation, 
the smoke particles roll over each other and execute a 
rotatory motion in every section of the ring. These 
motions take place from the interior towards the ex- 
terior of the ring, in the direction of the motion of 
translation, so that the entire mass of air, or of the smoke 


CONSTITUTION OF MATTER. 327 


which forms the ring, revolves continually round a 
circular axis, which forms, as it were, the nucleus of the 
ring. There is this remarkable fact in this rotatory 
motion, that all the particles which are situated upon 
one of the curves which can be drawn in each section of 
the ring are indissolubly tied down to their circular paths, — 
and can never quit them; so that the whole mass of the 
vortex ring will be always formed of the same particles. 
This theorem was proved by Helmholtz in 1858. This 
eminent physicist has analysed the vortex motions 
which would exist in a perfect fluid free from all friction. 
He has proved that in such a medium vortex rings, 
bounded by a system of vortex lines,! are formed of an 
invariable quantity of the same liquid molecules, so 
that the rings can move, and even change their form, 
without the connexion of their constituent parts ever 
being broken. They will continue to revolve, and 
nothing will be able to separate them, divide them, or 
destroy them. Those existing in the liquid will exist 
there for ever, and new ones can only be excited in it by 
a creative act. 

The smoke rings of which we have spoken above 
would give a perfect representation of these liquid vortex 
rings, if they were formed and moved in a perfect fluid. 
They are not so; but such as can be formed can serve 
for the demonstration of some properties of matter in 
vortex motion. They are endowed with elasticity and 
can change their form. The circle is their position of 
equilibrium, and, when their form is altered, they 
oscillate round this position, and finally reassume the 

' * Wirbelfaden und Wirbellinien.’ 


328 THE ATOMIC THEORY. 


circular form. But if we try to cut them they recede 
before the knife, or bend round it, without allowing 
themselves to be injured. They give, therefore, a 
material representation of something which would be 
indivisible. And when two rings meet each other 
they behave like two solid elastic bodies; after the 
impact they vibrate energetically. It is a singular fact 
that when two rings are moving in the same direction, 
so that their centres are situated upon the same line 
and their planes perpendicular to this line, the hinder 
ring contracts continually, whilst its velocity increases ; 
the ring in advance, on the contrary, expands, and its 
velocity decreases until the other has passed it, when 
the same action recommences, so that the rings alter- 
nately pass through each other. But, through all these 
changes of form and velocity, each preserves its own 
individuality, and these two circular masses of smoke 
move through the air as if they were something per- 
fectly distinct and independent. These curious experi- 
ments were made in England.! 

Helmholtz, therefore, has discovered the funda-_ 
mental properties of matter in vortex motion, and Sir 
William Thomson has stated: This perfect medium 
and these vortex rings which move through it represent 
the universe. A fluid fills all space, and what we call 
matter are portions of this fluid which are animated 
with vortex motion. There are innumerable legions of 
very small fractions or portions, but each of these por- 
tions is perfectly limited, distinct from the entire mass, 


1 P, G. Tait, Lectures on some Recent Advances in Physical Science. 
London, 1876. 


CONSTITUTION OF MATTER. 329 


and distinct from all others, not only in its own sub- 
stance, but in its mass and its mode of motion—quali- 
ties which it will preserve for ever. These portions are 
atoms. In the perfect medium which contains them 
all, none of them can change or disappear, none of 
them can be formed spontaneously. Everywhere atoms 
of the same kind are constituted after the same fashion, 
and are endowed with the same properties. It is well 
known, in fact, that the atoms of hydrogen vibrate 
exactly in the same periods, whether we heat them in a 
Geissler’s tube, observe them in the sun, or in the most 
distant nebula. 

Such is, in a few words, the conception of vortex 
atoms. It accounts, in a satisfactory manner, for some 
properties of matter, and of all the hypotheses upon the 
nature of atoms it appears to be the most probable. 
We see also that it permits the revival of the ancient 
hypothesis of the unity of matter, and in a more 
acceptable form than that of Prout’s hypothesis. Is 
the idea absolutely new? No; it was originally con- 
ceived by Descartes. So far is it true that, when the 
perpetual, and perhaps insolvable, problem of the consti- 
tution of matter is discussed, the human mind seems 
to turn in a cirele, the same ideas lasting for ages, and 
being presented under fresh forms to the highest intel- 
lects who have endeavoured to solve this problem. But 
is there no difference among these great intellects in 
their manner of working? Most certainly: some, more 
powerful perhaps, but bolder, have proceeded by in- 
tuition; others, better armed and stricter, by induc- 
tion. Here lies the progress and the superiority of 

15 


330 THE ATOMIC THEOPY. 


modern methods, and it would be unjust to pretend 
that the important efforts, of which we have had strik- 
ing testimony, have not made an advance~in this 
difficult problem which was impossible to Lucretius 
and even to Descartes. 

One word more before concluding. 

We have been able to see, from the preceding, that 
atoms are limited portions of matter in motion, what- 
ever may be the idea which we take of their nature 
and of their form. Since heat itself is a mode of 
motion, it results that thermo-chemical facts agree 
perfectly with the atomic hypothesis: they follow from 
it in some manner as a natural consequence. It is, 
therefore, useless to attempt to bring forward in oppo- 
sition to the hypothesis of atoms considerations drawn 
from thermo-chemistry, as furnishing a more solid 
foundation for molecular dynamics. Far from being 
opposed, these notions are correlative. The forces 
which are considered in mechanics must emanate from 
something, and they must be applied to something. 
In chemistry we suppose that they emanate from and 
are applied to imperceptible but limited and definite 
particles, which represent the fixed proportions accord- 
ing to which bodies combine. We call these particles 
atoms, endeavouring to interpret the modern and 
definite notion of fixed and multiple proportions by 
weight and by volume, by an ancient hypothesis which 
preserves the character of an hypothesis even under its 
altered form. 

- Does this mean that this hypothesis gains our 
acceptance because it explains so many facts in che- 


CONSTITUTION OF MATTER. 331 


mistry and physics? By no means. In its present 
form it is far from being perfect, and if it interprets in 
a wonderful manner certain phenomena of weight and 
measure, which are in reality the very foundation of 
chemistry, it leaves other phenomena in the shade. 
The properties of elementary and compound bodies are 
probably dependent upon the innate nature of atoms, 
upon their form and their mode of motion. But these 
matters are uncertain and unknown. That is the reason 
why, with our imperfect notions of the very essence of 
atoms, the theory cannot predict the form of compounds 
or their properties. These are matters of experiment. 
Now, a perfect theory ought not only to guide experi- 
ment, but to anticipate it. 

But, whatever may be the kind of hypothesis under 
discussion, one point is definitely gained, viz. the 
notation which is called atomic, since a name must be 
given to it, but which is independent, up to a certain 
point, of the hypothesis which it recalls. 

The present notation is founded upon facts. It sums 
up and reconciles in some manner the most important 
discoveries relating to chemical combinations—those, 
namely, of Richter, Dalton, and Gay-Lussac. It rests 
especially upon a rigorous application of the law of 
volumes discovered by the latter and interpreted by 
Avogadro and Ampére. And, when the law of volumes 
is at fault, owing to the fixity of the elements or of 
their compounds, we have recourse to the law of specific 
heats or the law of isomorphism for the determination 
of atomic weights and for the construction of formule. 

The notation which derives its name from the 


332 THE ATOMIC THEORY. 


atomic hypothesis rests, therefore, upon the sure founda- 
tion of experiment. The same may be said of the con- 
siderations upon atomicity. They are founded upon 
the fact that the forms of combinations are various, as 
we have learnt, in the first place, from the discovery of 
multiple proportions by Dalton, and, in the second place, 
from the discovery by Gay-Lussac of the relations by 
volume according to which gaseous bodies combine— 
relations which are simple, but not identical for dif- 
ferent gases. The considerations, therefore, upon the 
valency or the combining value of elements would 
survive the hypothesis of atoms if the latter were one 
day replaced by a more general hypothesis. But this 
day has not arrived ; it is useless to attempt to discredit 
the former as long as it proves itself productive. Its 
fertility and its power are clearly manifested in the in- 
cessant progress of the science. It has thrown light 
upon the most recent discoveries, as well as been, since 
the time of Dalton, its immortal author, the most 
perfect instrument in the most profound theoretical 
conceptions and the safest guide in experimental re- 
searches. 


APP BN DEX. 


NOTE I. 


WATER OF CRYSTALLISATION, 


THE combination with water of crystallisation is a phenome- 
non both of a chemical and of a physical order : of a chemical 
order, since the chemical force, elective affinity, plays a part 
in the phenomenon, and of a physical order, since it is 
connected with external form and change of state. Let us 
consider salts, properly so called. Their hydration or 
their degree of hydration is determined by the nature of the 
base, rather than by the nature of the acid; for we know 
that all the sulphates, for example, do not crystallise with 
the same quantity of water, and that two very similar sul- 
phates—those of potassium and of sodium, for instance—can 
exist one in the anhydrous, and the other in the hydrated 
condition. If, therefore, it is the nature of the base which 
determines the degree of hydration, at least in certain 
salts, it seems that it is the base also, or rather the metal, 
which attracts the molecules of water. This attraction may 
be due to the development of supplementary atomicities in 
the metal of the salts and in the oxygen of the water. We 
are now upon a ground riddled with hypotheses. For 
brevity’s sake, therefore, I will take a single example. 
Copper sulphate crystallises with five molecules of water. 
The copper becoming quadrivalent can attract these mole- 
cules, supposing the oxygen of the water itself to have 


334 APPENDIX. 


become quadrivalent, and we may conceive that the five 
molecules can be joined together so as to leave at the end of 
the chain two free atomicities by which the system is 
attached to the copper. This idea is expressed in the 
following formula :— 


OrH, 
a OH, 
ONS Ue | 
ei Aaebiie 6) 5 F 
a 
“OH, 


The molecules of water were free ; they are now riveted to 
one another and to the sulphate of copper: they have lost 
something, and changed their state by becoming solid, and 
this double condition has given rise to a disengagement of 
heat. And let us notice that this chain of molecules of 
water can be increased, so to speak, at will. In ferrous 
sulphate, for instance, it is increased by two links. Lastly, 
the molecules, especially when they are numerous, can fix 
themselves upon several polyatomic elements in the same 
compound. This might easily be developed, but I stop: I 
am satisfied with having stated the idea and given an 
example. 


NOTE II. 


THE CONSTITUTION OF DOUBLE SALTS. 


Let us take as an example the double sulphates of the 
magnesium series. In order to explain the attraction which 
sulphate of magnesium exercises upon sulphate of potassium, 
several hypotheses may be brought forward. The magnesium 
becomes quadrivalent, and exchanges two valencies either 


APPENDIX. 335 


with two atoms of oxygen, or with the atom of hexatomic 
sulphur of the molecule of sulphate of potassium. 

The two molecules are united by the sulphur of one to 
the oxygen of the other, or, which appears less probable, 
by the sulphur of one to the sulphur of the other, or by the 
oxygen of one to the oxygen of the other, the atoms of 
oxygen becoming quadrivalent. It is not necessary to 
develope this at length by formule which are moreover easy’ 
of construction. I here give two of these formule, and only 
add that such combinations between molecules rich in 
oxygen appear to me to be caused rather by the union, that 
is to say, by the affinity of heterogeneous atoms, than by the 
union of atoms of the same nature, as sulphur with sulphur 
and oxygen with oxygen. 

eo, DN puaice " wis A EARS 
OF NO hank OC OK 


Magnesium sulphate. Potassium sulphate. 


ry iv 
ONO rel eNgurn 
PP EP Ie SO 
Double sulphate. 
Here two atoms of oxygen of the potassium sulphate 


have become quadrivalent, as well as the magnesium of the 
molecule of magnesium sulphate. 


(2.) OK 
O. | O 
SO,Mg”+80,K,= |S =Mgr Sof 
ah tO No 
| 
OK 
Magnesium Potassium Double sulphate. 


su!phate. sulphate. 


Here magnesium, becoming quadrivalent, is attached to 
the hexatomic sulphur of the molecule of potassium sulphate. 

I will not dwell at greater length upon this view, since it 
is entirely hypothetical. 


336 APPENDIX. 


NOTE III. 


THE ISOMERISM OF THE AMYL ALCOHOLS, 


In order to give an idea of the facility with which the 
theory of atomicity interprets and foretells cases of isomer- 
ism, we will here discuss at greater length the isomerism 
of the amyl alcohols. We were unwilling to introduce it 
into the text, in order to avoid complicating our explana- 
tion. 

Primary, secondary, and tertiary alcohols are known 
(see p. 296). The normal amyl alcohol, discovered by Lie- 
ben, and the amyl alcohol of fermentation are both pri- 
mary. The fermentation alcohol rotates a ray of polarised 
light to the left. A. Le Bel has recently discovered the 
amyl alcohol which possesses a dextro-rotatory power. 
Pasteur made known the inactive variety, which is a mix- 
ture of the dextro-rotatory alcohol with the levo-rotatory 
alcohol. 

The following formule express the constitution of these 
alcohols :— 


CH, CH, CH, CH, CH, 
| ied 
CH, CH CH, CH,-—-U--UH. 
| 
CH, CH, CH—CH, CH,.0H 
CH, CH,.0H CH,.0H 
| 
CH,.0H 
Normal amyl Amy] alcohol Unknown. Unknown. 
alcohol. of fermentation. 


Comparing the constitution of these alcohols with that of 
ordinary alcohol, we may regard the two first as ethyl 
hydrate in which one atom of hydrogen of the group CH; is 


-_— 


APPENDIX. 337 


replaced by propyl or by isopropyl, the third as ethyl hydrate 
in which one atom of hydrogen is replaced by ethyl, and 
another by methyl, and lastly, the fourth as ethyl hydrate in 
which three atoms of hydrogen are replaced by three methyl 
groups; this last alcohol corresponding to the trimethylacetic 
acid of Boutlerow. ‘This point of view is expressed in the 
following formulee :— 


I. il. IIL. IV. 
: /CH, 
CH, CH,(0,H,) CHOaHL) CHC — O(CH)s 
2tts 
CH,.OH CH,.0H CH,.0H CH,.0OH CH,.0OH 
Ethylic Propyl- Isopropyl- Methyl-ethyl- Trimethyl-ethylic 
hydrate. Sree ethylic hydrate. ethylic hydrate. hydrate. 
1ydrate. 


The secondary amyl alcohols are three in number, viz.— 


V vI. vit 
CH, CH, CH, CH, 
| ree | 
CH, OH CH, 
CH, CH.OH CH.OH 
CH.OH CH, CH, 
| 
CH, CH, 
Boiling point 120°, Boiling point 108°, Boiling point . 
Wurtz. Wichnegratsky. 116°-117°, Saytzew. 


These secondary amyl alcohols can be regarded as de- 
rivatives of methyl alcohol in which two atoms of hydrogen 
are replaced either by propyl and methyl, or by isopropy] 
and methyl, or by two ethyl groups. 

We thus have the simplified formule :— 


Vv. VI. vil. 
C,H /(C,H,) /0,H 
H vA kK oe f peat 26 
ea aN ale eH) Sh See 
O OH OH OH 
Methylic Propyl-methyl- Isopropyl-methyl- Diethyl-methylic 
hydrate methylic hydrate methylic hydrate hydrate 


(carbino!), (propyl-methyl carbinol). (isopropyl-methyl (diethyl carbinol), 
carbinol). 


338 APPENDIX. 


Lastly, a tertiary amyl alcohol is known : it is the body 
which I have described under the name of amylene hydrate. 
It contains two methyl and one ethyl groups. 


vul. 


| ; or be . 
CH,—C.0OH OH 

| 

JH, 

| 

CH, 


Tertiary amyl alcohol. 


We have given all these formule in order to show with 
what facility the theory of atomicity foretells, limits, and 
interprets the most complicated cases of isomerism. The 
principles here developed may be applied to many other 
examples. 


NOTE IV. 


THE ACTION OF HEAT UPON GASES, 


We have pointed out in the text the manner of action of 
heat upon the molecules of a solid body. We think it use- 
ful to follow up this point by analysing the case of a gaseous 
body. We know that the heat absorbed by gases produces 
different effects, whether we heat it under constant pressure 
or under constant volume. In the first case— 

(i.) It increases the external work corresponding to dilata- 
tion and to the pressure supported by the gas ; 

(ii.) It increases the energy of the progressive rectilinear 
molecular motion ; 

(iii.) It increases the energy of the atomic motion, and 
performs certain work within the molecule when the mole- 
cule is composed of many atoms. 

In the second case, when the gas is heated under con- 


Ea 


APPENDIX. 339 


stant volume, the first effect is nil. The second and the 
third are produced, but nothing proves that the internal 
work is the same under constant pressure and under con- 
stant volume. This work disappears in the case of mon- 
atomic gases, such as mercury vapour (p. 121). It should 
increase with the number of atoms in the molecule. 

The total energy of a gaseous molecule is composed of 
the energy of the progressive molecule motion and of the 
atomic energy (Kinetic and Potential). Clausius admits 
that a relation exists between the total energy H and the 
energy of the progressive motion kK, and he expresses this 
relation by the following equation :— 


k being the ratio of the specific heats © (p. 122). In the case 
c 


of mercury vapour H=K, k=1:666. In the case of poly- 
atomic gases, the values of & become smaller, for the gases 
H,, O., and N, falling between 1:395 and 1°413. These 
values decrease as the number of atoms in the gaseous mole- 
cule increase. . 

The absolute zero would correspond to the cessation of 
the molecular and atomic motions. The temperatures of a 
gas increase proportionately with the kinetic energy of its 
molecules ; or again, since the masses remain constant, with 
the squares of the molecular velocities. The heat contained 
in a gaseous mass is represented by the sum of the kinetic 
energies of its molecules, 


ACE 


ACETIC anhydride, 100 

Affinity, 224; Berthollet on, 7; 
distinguished from atomicity, 
204 

Alcohols, primary, 
and tertiary, 295 

Aluminium, atomicity of, 221 

Ammonium, chloride, 114; car- 
bamate, 115; sulphydrate, dis- 
sociation of, 114 

Ampére, 38 

Amy] alcohol, isomers of, 336 

Amylene, dissociation of bromide, 
111; isomers of, 291 

Anaxagoras, 305 

Anhydrides, constitution of, 86,99 

Antimony, sesquioxide, constitu- 
tion of, 227 

Apatite, 180 

Arsenic, constitution of acids of, 
241; density of, 69, 120; 
organo-metallic compounds of, 
272 

Atomic constitution of mole- 
cules, 298 

Atomic volumes, 187 

Atomic weights of Berzelius, 62; 
how determined, 61; of Dal- 
ton, 25, 29; present system of, 
92 

Atomicities, supplementary, 232, 
248 

Atomicity, a relative property, 
226; definition of, 19, 226; 
distinguished from affinity, 


secondary, 


CAN 


308 ; historical development 
of, 197; increase of, 225; of 
radicals, 262; variations ex- 
plained, 238 

Atoms and molecules, Avogadro 
on, 38 

Atoms, double, of Berzelius, 65 

Avogadro, 36 

Avogadro and Ampére, law of, 
42, 95 ; exceptions to, 110 

Azobenzene, 219 


BERGMAN, 3 

Bernouilli, 314 

Berthelot, 204, 287 

Berthollet on affinity, 4, 7, 21 

Berzelius, atomic weights of, 62; 
on atomic weights, 43; on 
double atoms, 64 

Bivalent metals, 179 

Boisbaudran, Lecoq de, 162, 166, 
168 

Boron, specific heat of, 127 

Brodie on copper hydride, 207 ; 
on decomposition of peroxides, 
208 ; on graphitic acid, 129 

Bunsen on molecular volatilities, 
137 


CADMIUM, density of, 121 

Cahours, 271 

Calomel, dissociation of, 115% 
formule of, 136 

Cannizzaro, 88 


J42 INDEX. 


CAR 


' Carbon, atomicity of, 211, 234; 


monoxide, 235; specific heat 
of, 127 

Cauchy, 321 

Chloral hydrate, dissociation of, 
115 

Chlorine, variable atomicity of, 
229 

Chloroplatinates, 253 

Chromic compounds, constitution 
of, 222 

Chromium sesquioxide, formula 
of, 60 


Classification, of elements, 150, | 


155; natural, 176, 215, 265 
Clausius, 316 
Clerk Maxwell, 317 , 
Constitution of bodies, 259; of 
matter, 305 
Constitutional formule, 291, 298 
Cooper, 235 
Copper, hydride, 207 
Coppet, 254 
Crystallisation, water of, 181, 
248, 253, 333 
Cuprous compounds, constitution 
of, 223 
Cyanogen, 236, 263 


DALTON, 2, 23, 64, 77, 316 

Davy, Sir H., 33, 48 

Democrites, 305 

Density, 163; atomic weights 
calculated from, 101, 104; of 
vapours, abnormal, 69 

Descartes, 306 

Deville, 112, 113, 116 

Deville and Troost, 102 

Diatomic gases, 71, 119, 207 

Dimorphism, 57 

Dissociation, 111 

Dulong and Petit, 52 

Dumas, 51, 67, 150, 190 


ELEMENTARY vapours, constitu- 
tion of, 118 


HYD 


‘Elementary volumes’ of Berze- 
lius, 44 

Elements, atomic constitution of, 
119, 205 

Equivalence, 76 

Equivalents, 33 

Erlenmeyer, 115 

Ether, 99; of space, 309 

Ethyl, 100, 273 ; cyanide, 276 

Ethylene, constitution of, 266 


FARADAY, 323 

Ferric compounds, constitution 
of, 221 

Fischer, 19 

Frankland, 200, 291 

Friedel, 220, 231, 253 


GALLIUM, discovery of, 162, 166 

Gases, diatomic, 71, 207; friction 
of, 317; kinetic theory of, 
315, 317; velocity of molecu- 
lar motion in, 316 

Gaudin, 254 

Gautier, 236 

Gay-Lussac, 34, 64, 77 

Gerhardt, reform in atomic 
weights, 80 

Glycerine, compounds of, 89, 
200, 185; constitution of, 283 

Glycol, 89; discovery of, 202 

Gmelin, objections to atomic 
weights of Berzelius, 71 

Graphite, 128 


HEAT, its action on bodies, 311, 
358 

Helmholtz, on vortex motion, 327 

Hexatomic gases, 71 

Higgins, 27 

Hofmann, 323 

Horstman, 114 

Hydrocarbon radicals, atomicity 
of, 201, 213 

Hydrocarbons, saturated, 213 

Hydroxyl, 263 


INDEX. 


IoD 


IopINS, variable atomicity of, 
230 

Tron, atomicity of, 221, 233 

Isomerism, Berthelot on, 287 

isomers of amyl alcohol, 336 ; 
of amylene, 291; of trichlor- 
hydrin, 286 

Isomorphism, definition of, 145 ; 
discovery of, 56; formule de- 
termined by, 139; influence 
on atomic weights, 60, 138 


JOULE, 194 


KANT, 306 

Kekulé, 211, 213 

Kinetic theory of gases, 315 

Kirchhoff and Bunsen, 323 

Kolbe, 295, 336 

Kopp, on molecular heats, 131 ; 
on atomic volumes, 189 

Kundt and Warburg, 121 


LACTIC acid, 302 

Laurent, 84 

Lavoisier, 4, 18 

Law of Avogadro and Ampére, 
36, 42, 95; of Dalton on pres- 
sure of mixed gases, 316; of 
definite proportions, 3; of Gay- 
Lussac, 34, 95; of Dulong and 
Petit, 53; of isomorphism, 56; 
of Mariotte, 315, 320; of Men- 
delejeff, 158 ; of multiple pro- 
portions, 24; of proportion- 
ality, 11; of Proust, 10 

Le Bel and Van’t Hoff, 300 

Lead tetrachloride, 249 

Lesage, 309 

Lorschmidt, 319 


MARIGNAC, 114, 146 

Mariotte’s Law, 315, 320 

Marsh gas, a type of compounds, 
300 


343 
PER 


Matter, constitution of, 
discontinuity of, 308 
Mendelejeff, 154 
Mercurous compounds, constitu- 
tion of, 223; formule deter- 
mined by specific heat, 135 
Mercury, density of, 67, 121 ; 
specific heat of vapour, 122 
Methyl, 100, 273; carbylamine, 


305 ; 


236; cyanide, 236; oxide 
chlorhydrate, 231, 250 
Meyer, Lothar, 163, 167 
Mitscherlich, 56 
Molecular combinations, 248; 


diameters, 320; dissymmetry, 
301; heats, 130; symmetry, 
300; volatilities, 136 

Molecular volumes of liquids, 
189; of solids, 194 

Molecules, integral and element- 
ary, 38; mean velocity of, 
316; number in wnit volume 
of gases, 322 

Monatomic gases and vapours, 
71 


NASCENT state, 208 

Naumann, 115 

Neutrality, permanence in mu- 
tual decomposition of sas+s, 13 

Nickles, 249 

Nitrogen, atomicity, 227 ; binox- 
ide, Dalton on, 23; chloride, 
210; constitution of acids of, 
241; substitution derivatives, 
219 


ODLING, 90, 199, 203 

Organo-metallic radicals, ato- 
micity, 271 

Oxygen, atomicity, 218, 231 

Ozone, 119 


PERIODIC Law, 158, 163, 170 
Peroxides, constitution of, 219, 


344 
PHO 

264; mutual decomposition of, 
208, 210 

Phosphorus, constitution of 
acids, 241, 280; density, 68, 
120; dissociation of penta- 
chloride, 112 

Plateau, 323 

Playfair, 194 

Poisson, 309 

Polyatomic metals, similarity to 
organic radicals, 183 

Polybasic acids, 76, 199 

Proportional numbers, 33 

Proportionality, law of, 11 

Proust, 5, 10 

Prout, 49 

Pyrophosphoric acid, 243, 280 


RADICALS, alcoholic, 100, 263, 
273; atomicity of, 262; inor- 
ganic, atomicity of, 279 

negnault, 125, 132, 141 

Richter, 12, 22 

Riidorff, 254 


SALTS, constitution of, 83; dou- 
ble salts, 253, 334; hydrated, 
formule of, 181 ; permanence 
of neutrality on mutual de- 
composition, 13 

Schelling, 306 

‘Series of masses’ of Richter, 15 

Sesquioxides, constitution of, 
222; formule of, 61, 197 

Silicon, atomicity of, 220; specific 
heat, 127 

Specific heat, 52; table of, 124; 
abnormal, 127; of gases under 
constant volume, 121; of 
liquids, 129 

Stas, 51 

Sulphur, density, 68,119; varia- 
tions in atomicity, 231 


INDEX. 


WOL 


Sulphuric acid, constitution of 
199, 269 ; dissociation of, 113 


TARTARIC acid, 302 

Temperature, measured by 
energy of molecular motion, 
315 

Tetratomic gases, 71 

Thomson, 28, 64 

Thomson, Sir W.,on molecular 
motion, 324; on vortex motion, 
328 

Titanium, atomicity, 220 

Trichlorhydrin, isomers of, 285 

Trivalent radicals, 203 

Troost, 102, 116 

Tungsten, variations in atomicity 
of, 233 

Types, 85, 198, 260; condensed, 
89, 199 


UREA, 237 


Vacuum, 306 

Valency, 197 

Van der Waals, 320 

Vapour density, abnormal, 70, 
120; determinations of atomic 
weights from, 101 

Volumes, law of, 35 

Vortex-atoms, 329 

Vortex motion, 308 


WANKLYY, 113 

Warburg, 121 

Water of crystallisation, 
248, 253, 333 

Water type, 85, 198 

Wenzel, 11 

Williamson, 85, 99, 198 

Wollaston, 28, 64 


181, 


as 


spveestss 
aeeieebestesssee 


ana 


eo 
- | ae 
| t 

r 


seersauepses 


gogsece 


Lt naan 


fm si 4 
JA 
sigees 
ae 


St 


Bea 


ferrite 


Rp. ee 


S20sceeaeageueeeanue 


Hae 


azsstastarets sere 


ie 


ee ee 


Soe ceseeuEs 


,csahSe 


sssHeit 


SeeGGe0 G2ceeebera es 


FESVERAARKMAAKA TK EE OVP ASAP « 
aSGR SSP RRT wo L 
Seeessre=aasaat 


st 


Bd 
7 ; 
, 


% 
wt 


7 


T TUT IT LL 
DA SPSE SSE RS eae 
snece 


SasRei 


Suh Nel iraafens 


r a 


To tT ted 
BE&¢eeSs 


a 
Au 


ya 


BS: 


a 


Et 
Gapsescess 
jd 


4 
a! 
¢ 
* 
Re 
i 


286 02a8 
C4 


CCE ea 
PET 


EMS Hie 
a 
Ss 
aS 
eeu 
e 

a 

Lf 

| 

RBS 
[| 


senne @ 
pe 
a 


genees 
eee 


aR en ae 
Seewe 
EZ ERG 
Beak 


SRERESE 


q 


SSSeeE285 
| 


PREP E 


PEE 
amas 


Gsssguctrat 
rt 
HE 


ae 
pi tt 


sete 


are 


BS 
mr te 
see 


se seats — Snr 


: i 


pune 


Santa | seekeees opeetees pase Steet 


eft aaa i 4 
tt 


+ 
pasateeesseas | 


co eeiaeses 
He sage sitet : 


ae FE 


ee aes 
seisrietee (ae a 
nae 


-_—_ 


7 at ..] 
= : é 
5 ee ee a he 
Oe ai BS aE 
Span mai ai 


i 


E saaues HH sede aie erin aE 


Steet ait 


ace aneen? east eae sen 
Saaaccumesesdaneas Z ane | att ; 


= Seeereaeerina 
ages a — i: 


gene saga8 ; “at ae 
: a He eH 5 Bi: 


aa F LT GI eExs a ; HEE fete Soe 
EES EEAS HE ERSESIEE CE etaeat toeertoset sori Se 


ehren: der Chemic . 


Gist 


be 


erica 


S odernen: 


i aii 


: faite eee 


is 


r ane f a if. 
: dgsgtessecs ae { mt ema ae. + hile Peet . 
asitie ui | ane 


Lay 2 ae sunk 


i 


bo 


oO st oR 


28. 


INTERNATIONAL SCIENTIFIC SERIES. 


NOW READY. In 12mo and bound in cloth. 


FORMS OF WATER, in Clouds, Rain, Rivers, Ice, and Glaciers. By Prof. 
Joun TYNDALL, $1.50. 

PHYSICS AND POLITICS; or, Thoughts on the Application of the Principles 
of “ Natural Selection” and * Inheritance” to Political Society. By WATER 
Bacenor. $1.50. . 


. FOODS. By Epwarp Smrru, M. D., LL. B., F. B.S. $1.75. 


MIND AND BODY. By Atexanper Baty, LL.D. $1.50. 


. THE STUDY OF SOCIOLOGY. By Hersert Spencer, $1.50. 
-THE NEW CHEMISTRY. By Prof. Josian Pb. Cooker, Jr., of Harvard 


University. $2.00. 
THE (eka hel OF ENERGY. By Prof. Batrour Stewart, LL.D., 
1.5 


F.R.S. , 
_ ANIMAL LOCOMOTION; or, Walking, Swimming, and Flying, with a bis- 


sertation on Aéronautics. By J. B. Perrigrew, M.D. Illustrated. $1.75. 


._ RESPONSIBILITY IN MENTAL DISEASE. By H. Mavpstey, M D. 


$1.50. 
. THE SCIENCE OF LAW. By Prof. Suetpon Amos. $1.70. 
. ANIMAL MECHANISM. A Treatise on Terrestrial and Aérial Locomotion. 


By E. J. Marry. 117 Illustrations. $1.75 


. THE HISTORY OF THE CONFLICT BETWEEN RELIGION AND 


SCIENCE. By Joun Winuiam Draper, M.D., LL.D. $1.75. 


.THE DOCTRINE OF DESCENT, AND DARWINISM. By Prof. Oscar 


Scumrpt, of Strasburg University. $1 


50. : 
. THE CHEMISTRY OF LIGHT AND PHOTOGRAPHY. By Dr. Her- 


MANN VoGEet. 100 Illustrations. $2.00. 


. FUNGI; their Nature, Influence, and Uses. By M. C. Cooke, LL. D. ~'Ed- 


ited by M. J. Berxetey. 109 Illustrations. $1.50. 


._ THE LIFE AND GROWTH OF LANGUAGE. By Prof. W. D. Wuitney, 


of Yale College. 


$1.50. 
. MONEY AND THE MECHANISM OF EXCHANGE. By W. Sranrey 


Jevons. M.A., F.R.8. $1.75. 


$ 
_THE NATURE OF. LIGHT, with an Account of Physical Optics. By Dr. 


E. Lommet, Professor in the University of Erlangen. 88 Llustrations and 
a Plate of Spectra in Chromo-lithography. $2.00. 


. ANIMAL PARASITES AND MESSMATES. By M. Van Benepen, Pro- 


fessor of the University of Louvain. 83 Illustrations. $1.50. 


.ON FERMENTATIONS. By P. Scnirzenpercer, Director at the Chemical 


Laboratory at the Sorbonne. 28 Illustrations. $1.50. 


_THE FIVE SENSES OF MAN. By J. Bernstern, O. O. Professor in the 


University of Halle. 19 Illustrations. $1.75. 


_THE THEORY OF SOUND IN ITS RELATION TO MUSIC. By Prof. 


owe of the Royal University of Rome, Numerous Wood- 
cuts, $1.5 


0. 
STUDIES IN SPECTRUM ANALYSIS. By J. Norman Lockyer. Iilustra- 


tions. $2.50. 


_A HISTORY OF THE GROWTH OF THE STEAM-ENGINE. By Rosert 


H. Tavrsron, A. M., ©. E., Professor of Mechanical Engineering. 163 Llus- 
trations. $2.50. 


. EDUCATION AS A SCIENCE. By Atexanper Barn, LL. D., Professor of 


Logic in the University of Aberdeen. $1.75. 


. MODERN CHROMATICS, with Applications to Art and Industry. By OapEN 


N. Roop, Professor of Physics in Columbia College. 130 original Mlustra- 
tions. $2.00 


. THE HUMAN SPECIES. By A. Dr Quatreraces, Professor of Anthropology 


in the Museum of Natural History, Paris. $2.00. 
THE CRAYFISH: An Introduction to the Study of Zodlogy. By Prof. T. 
H. Huxuery. §&2 Illustrations. $1.75. ; 


For sale by all booksellers. Any yolume sent by mail, post-paid, to any address 


in the United States, on receipt of price. 


D. APPLETON & CO., Publishers, 1,8, & 5 Bond St., New York. 


NEW) “BOOKS: 


THE BRAIN AS AN ORGAN OF MIND. By H. CHARLTON BASTIAN, 
Professor of Anatomy and Clinical Medicine in University College, Lon- 
don; author of *‘ Paralysis from Brain Disease.’ With numerous Illus- 
trations. One vol., 12mo, 708 pages. Cloth. Price, $2.50. 

‘“The fullest scientific exposition yet published of the views held on the sub- 
ject of psychology by the advanced physiological school. It teems with new 
and suggestive ideas; and, though the author displays throughout his customary 
boldness of speculation, he does not allow himself to be carried away so freely 
Te old by his own exuberant wealth of ‘scientific imagination.’ ’’—Zondon 
Atheneum. 


Second Volume’ of 


COOLEY’S CYCLOP4ZDIA OF PRACTICAL RECEIPTS. Cooley’s 
Cyclopedia of Practical Receipts and Collateral Information in the Arts, 
Manufactures, Professions, and Trades, etc., etc. Sixth edition, revised 
and partly rewritten by Professor RicHarp V. Tuson. Volume two, com- 
pleting the work, now ready. 8vo, 1,796 pages (complete). Price, $4.50 
per volume. 


A HISTORY OF PHILOSOPHY IN EPITOME. By ALBERT ScHWEG- 
LER. Translated from the first edition of the original German by Julius 
H. Seelye. Revised from the ninth German edition, containing Important 
Additions and Modifications, with an Appendix, continuing the History 
in its more Prominent Lines of Development since the Time of Hegel, by 
Benjamin T. Smith. 12mo, 469 pages. Cloth. Price, $2.00. 


A SHORT LIFE OF GLADSTONE. By C. HW. Jonsgs, author of ‘tA Short 
Life of Charles Dickens,” ‘t Macaulay,” etc. ‘tHandy-Volume Series.” 
Paper, 35 cents; cloth, 60 cents. 


LIVY. By the Rev. W. W. Capzs, M.A. Fifth Volume in ‘‘ Crassican 
WRITERS,’’ 16mo, flexible. Price, 60 cents. Previously published in the 
ecries: ‘* Milton,” ‘ Euripides,”’ ‘‘ Sophocles,”’ ‘* Vergil.’’ Uniform style. 
€0 cents each. 


EDUCATION: Intellectual, Moral, and Physical. By HERBERT 
SpeNcEeR. A new cheap edition of Herbert Spencer’s famous Essays on 
Education. One yol., 12mo, paper cover. Price, 50 cents. 


For sale by all booksellers; or any work sent by mail, post-paid, on receipt 
of price. . 


D. APPLETON & CO., Publishers, 
1, 3, & 5 Bonp Street, NEw York. 


NEW BOOKS. 


ATEMORIES OF MY EXILE. By Louis Kossotn. Translated from the 
original Hungarian by FERENcZ JAusz. One vol., crown 8yvo. Cloth. 
Price, $2.00. 

‘“*A most piquant and instructive contribution to contemporary history.”— 

New York Sun. 


‘*“ These * Memories’ disclose a curious episode in the inner life of English do- 
mestic politics.”— Zhe Nation. 


THE HISTORICAL POETRY OF THE ANCIENT HEBREWS. 
TransJated and critically examined by MicuaEL HEILPRIN. Vol. II. 
Crown 8vo. Cloth. Price, $2.00. 

“The notion has somehow got abroad that the scientific study of the Bible is 
inconsistent with the most tender reverence for its contents, or with their per- 
sistent fascination. But the reverence of Mr. Heilprin for the subject-matter of 
his criticism could hardly be surpassed; and, that it has not lost its power to 
interest and charm, his book itself is ample evidence, which will be reénforced 


by the experience of every intelligent reader of its too brief contents.”—New York 
Nation, July 24, 1879. 


HEALTH. By W. H. CorFie.p, Professor of Hygiene and Public Health at 
University College, London. 12mo. Cloth. Price, $1.25. 


FRENCH MEN OF LETTERS. Personal and Anecdotical Sketches of 
Victor Hugo; ALFRED DE MussEeT; THEKOPHILE GAUTIER; HENRI MuR- 
GER; SAINTE-BEUVE; GERARD DE NeERvAL; ALEXANDRE DUMAS, FILS; 
Emitz AuGIER; OcTAVE FEUILLET; VICTORIEN SARDOU ; ALPHONSE 
Davpet; and Emite Zona: By Maurice Mauris. Appletons’ ‘* New 
Handy-Volume Series.” Paper, 35 cents ; cloth, 60 cents. 


A THOUSAND FLASHES OF FRENCH WIT, WISDOM, AND 
WICKEDNESS. Collected and trauslated by J. DE Finop. One vol., 
16mo. Cloth. Price, $1.00. 

This work consists of a collection of wise and brilliant sayings from French 
writers, making a rich and piquant book of fresh quotations. 


‘** The book is a charming one to take up for an idle moment during the warm 
weather, and is just the thing to read on the hotel piazza to a mixed company 
of ladies and gentiemen. Some of its sayings about the first mentioned would 
no doubt occasion lively discussion, but that is just what is needed to dispel the 
often wellnigh intolerable languor of a summer afternoon.’’—Soston Courier. 


SCIENTIFIC BILLIARDS. Garnier's Practice Shots, with Hints to Ama- 
teurs. With 106 Diagrams in Colors. ' By ALBERT GARNIER. Oblong 
12mo. Price, $3.50. 


For sale by all booksellers; or any work sent post-paid to any address in the 
United States, on receipt of price. 


D. APPLETON & CO., Publishers, 
1, 3, & 5 Bonn Srreet, New York. 


Works of Alexander Bain, LL.D., 


PROFESSOR OF LOGIC IN THE UNIVERSITY OF ABERDEEN, 


LOGIC, 
DEDUCTIVE AND INDUCTIVE. New revised edition. 12mo. 
Cloth, $2.00. 


IL 
MENTAL SCIENCE: 


A Compendium of Psychology and History of Philosophy. 12mo., 
Cloth, $1.50. 


MORAL SCIENCE : 
A Compendium of Ethics. 12mo. Cloth, $1.50. 


ENG 
MIND AND BODY. 
The Theories of their Relations. (Forming a volume of “ The 
International Scientific Series.”) 12mo, Cloth, $1.50. 
Vv. 
THE SENSES AND THE INTELLECT. 
New edition. 8vo. Cloth, $5.00. . 


aig 
THE EMOTIONS AND THE WILL. 
Third edition. 8vo. Cloth, $5.00. 


VIL. 
EDUCATION AS A SCIENCE. 


(Forming a volume of “The International Scientific Series,’’) 
12mo,. Cloth, $1.75. 


‘*The work should become a text-book for teachers, not to be followed ser- 
vilely or thoughtlessly, but used for its suggestiveness.”— Boston Gazette. 


‘* Professor Bain is not a novice in this field. His work is admirable in many 
respects for teacher, parent, and pupil.’’—Phdladelphia North American. 


‘** A work of great value to all teachers who study it intelligently.”°—Boston 
Advertiser. 


D. APPLETON & CO., Publishers, 
1. 3. & 5 Bond StreET, NEw YorE. 


EMINENT MODERN SCIENTISTS. 


a re 


HERBERT SPENCER’S WORKS. 


Winet PRINOWPLES SG. cules ccceces cose $2 00 
PRINCIPLES OF BroLoay. 2 yols... 4 00 
PRINCIPLES OF PsYCHOLOGY. 2 vols. 4 00 
PRINCIPLES OF SocroLoey, Vol. I. 

Gp yr 7 Fe i 8 0 2 00 
CEREMONIAL INstitTuTIONS. Being 

Part LV. of the Principles of Soci- 

RGR Vai orate We cuits > pes 59's 3.08 ae 1 25 
PRR OMT ERTO Ss th wont. o's. = a0. 0-00 1 25 


PHILOSOPHY OF STYLE. 
DeEsorRIPTIVE SOctOLoey. 


> 


12mo. 


To Subscribers, for the whole work, per part, $3.50; single 
art, $4.00. Published in folio, with Tables. 


14 vols, 12mo. Cloth, $25.25. 


Stupy or Socrotoey. (International 


Scientific Series ic. sc. a. teeta $1 50 
HDUDATION Aca ance sk oe tess 1 25 
Discussions IN Scrence, PxHLoso- 

PHY; AND. MORAEA, oi; eee cay tv 2 00 
UNIVERSAL PROGRESS .... 22-0200. 2 00 
Essays: Moral, Political, and Ais- 

THOtIG..F alice tet ons coins We ok. ake 2 00 
BOOFAT) STATICH:: - day unschis stinw a 2 00 


Flexible cloth, 50 cents. 


Six Parts now ready, namely: 1 


nglish; 2. Ancient Americans; 3. Negritto and Malayo-Polynesian Races; 4. 


African Races; 5. Asiatic Races; 6. North and South American Races. 


CHARLES DARWIN’S WORKS. 


GRIGIN OF SPROIES. «<2: cccwcccse $2 00 
MB OMN TY OR NLAIN 6 c.0.0:0 sid croniewele os 8 00 
JOURNAL OF RESEARCHES.......... 2 00 
EEMOTIONAL EXPRESSION........... 8 50 
ANIMALS AND PLANTS UNDER Do- Sah 

0 


MESTIOCATION. 2 VOIS.........0000 


THOMAS H. HUXLEY’S WORKS. 


Man’s PLAcE IN NATURE......... $1 25 

ON THE ORIGIN OF SPECIES........ 1 00 

More Criticisms oN DARWIN, AND 
ADMINISTRATIVE NIHILISM....... 50 


A MANUAL OF THE ANATOMY OF 
VERTEBRATED ANIMALS. Illus’d.. 2 50 
A MANUAL OF THE ANATOMY OF IN- 
VERTEBRATED ANIMALS. Illus’d.. 2 50 
Lay Sermons, ADDRESSES, AND RE- 


11 vols., 12mo. 


Cloth, $24.00. 


INSECTIVOROUS PLANTS...........- $2 00 
CLIMBING. PLANTS Wu. ce so tnele cee 1 25 
ORCHIDS FERTILIZED BY INSECTS... 1 75 
FERTILIZATION IN THE YEGETABLE 
KANGDOMG a adcr sbie dt < cares atene 2 00 
PORMS! OF PLOWEES 4... cece occie 1 50 


11 vols., 12mo. Cloth, $18.00. 


CRITIQUES AND ADDRESSES........ $1 50 
AMERIOAN ADDRESSES............. 1 25 
PHYSIOGR APHY:« suttes's teueees sane 2 50 


ELEMENTS OF PHYSIOLOGY AND Hy- 
GIENE. By T. H. Huxley and W. 
es VOUMSNS. . a. een oe Aoi tae 

Tue CrayrisH: An Introduction to 
Zoblogy. (international Scientific 
Ol CL) Reus SRB D Onearaceoaadta Ae es) 


JOHN TYNDALL’S WORKS. 10 vols, 12mo. Cloth, $19.75, 


Heat as A MopE or Mortion..... $2 00 
RRR ial ig iG 4 > Sia nies ose 2 00 
FRAGMENTS OF SOTENCE........-.- 2 50 
LiGHT AND ELECTRIOCITY........... 1 25 


LESSONS IN ELEOTRIOITY..........- 


Hours or EXERcIsE IN THE ALPS.$2 00 


FARADAY AS A DISCOVERER....... 1 00 
On ForMS OF WATEBR............ 1 50 
RADIANS HAT YL .. eee. eee oe 5 00 
Srx LEOTURES ON LIGHT........... 1 50 


Banquet AT DELMONICO’s, paper, 50 cents; BeLtrast AppREss, paper, 50 cents. 


D. APPLETON & CO., Pus.isuxrs, 1, 3, & 5 Bonp Street, New Yorx«. 


WORKS OF HENRY THOMAS BUCKLE. 


i 


The Life and Writings of Henry 
Thomas Buckle. 


By Atrrep Henry Hurn. 12mo. Cloth, 


**The book deals with Mr. Buckle less as a philosopher than as aman... . 
Mr. Huth has done his part well and thoroughly.”—Saturday Review. 

‘*Mr. Huth has produced a striking and distinct portrait out of his materials, 
and he has done his work with a simplicity and modesty which are highly effec- 
tive.’—Pall Mall Gazette. 

‘*This work, we think, will revolutionize popular opinion about the philoso- 
pher.”’—London Daily News. 

‘Buckle was @ man whose story must excite interest and rouse sympathy.” 
— Scotsman. 


i: 


History of Civilization in England. 
2 vols., 8vo. Cloth, $4.00; half calf, extra, $8.00. 


‘* Whoever misses reading this book will miss reading what is, in various re- 
spects, to the best of our judgment and experience, the most remarkable book 
of the day—one, indeed, that no thoughtful, inquiring mind would miss reading 
for a good deal. Let the reader be as adverse as he may be to the writer’s philos- 
ophy, let him be as devoted to the obstructive as Mr. Buckle is to the progress 
party, let him be as orthodox in church creed as the other is heterodox, as dog- 
matic as the author is skeptical—let him, in short, find his prejudices shocked at 
every turn of the argument, and all his prepossessions whistled down the wind 
—still, there is so much in this extraordinary volume to stimulate reflection and 
excite to inquiry, and provoke to earnest investigation, perhaps (to this or that 
reader) on a track hitherto untrodden, and across the virgin soil of untilled fieldr, 
fresh woods and pastures new, that we may fairly defy the most hostile spirit, 
the most mistrustful and least sympathetic, to read it through without being 
glad of having done so, or, having begun it, or even glanced at almost any one 
of its pages, to pass away unread.’’—London Times. 

‘We have read Mr. Buckle’s volumes with the deepest interest. We owe 
him a profound debt of’gratitude. His influence on the thought of the present 
age can not but be enormous, and if he gives us no more than we already have 
in the two volumes of the magnus opus, he will still be classed among the fathers 
and founders of the Science of History.’’—New York Times. 

‘*Singularly acute, possessed of rare analytical power, imaginative but not 
fanciful, unwearied in research, and gifted with wonderful talent in arranging 
and molding his material, the author is as fascinuting as he is learned. His 
erudition is immense—so immense as not to be cumbersome. It is the result 
ofa long and steady growth—a part of himself.’-— Boston Journal. 


ITI. 


Essays. 


With a Biography of the Author. Portrait. 12mo. Cloth, $1.00; 
half calf, extra, $2.50. 


D. APPLETON & CO., Publishers, 1,3, & & Bond St, New York. 


. NEW BOOKS. 


A TREATISE ON THE PRACTICE OF MEDICINE, for the Use 
of Students and Practitioners. By Roberts BartHotow, M.A., M.D., 
LL. D., Professor of Materia Medica and Gereral Therapeutics in the Jef- 
ferson Medical College of Philadelphia. In one volume, 8vo, 853 pages. 
Cloth, $5.00; sheep, $6.00, 


STRATEGOS: A SERIES OF AMERICAN GAMES OF WAR, based upon 
Military Principles, and designed for the Assistance both of Beginners and 
Advanced Students in prosecuting the whole Study of Tactics, Grand Tac- 
tics, Strategy, Military History, and the Various Operations of War. Illus- 
trated with numerous Diagrams. To which is appended a Collection of 
Studies upon Military Statistics as applied to War on Field or Map. By 
CHARLES A. L. ToTrEN, First Lieutenant, Fourth United States Artillery. 
In two vols. Vol. I, Text and Appendices; Vol. II, Plates, Tables, and 
Statistics. Cloth. Price, $3.00. 


THE FORESTERS. A Novel. By BertHOoLD AvERBACH. Appletons’ ‘‘ New 
Handy-Volume Series.””> 18mo. Paper. Price, 50 cents. 


‘““The plot of this novel is slight, but skillfully told, the characters well em- 
phasized and graphically depicted. The action passes among rather a higher 
section of society than is usually chosen by Auerbach, and notably among the 
foresters, officials of the Public Forest Department—that body of whose learning 
and intelligence in preserving her forests Germany may be justly proud. It is 
readable as well as instructive, and possesses the further merit, rare in a German 
novel, of brevity..".—London Atheneum. 


COOPER’S “ LEATHER-STOCKING TALES” and ‘“‘SEA TALES.’’ 
New and remarkably Cheap Editions. 


THE LEATHER-STOCKING TALES. ByJ. FENtMoRE CooPER. Complete 
in one volume, 8vo. With Illustrations by F. O. C. Darley. In cloth, with 
gilt side and back, Price, $2.00. 


THE SEA TALES. By J. Fenrmore Cooper. Complete in one volume, 
8vo. With Illustrations by F.O.C.Darley. Cloth, with gilt side and back. 
Price, $2.00. 


For sale by all booksellers; or any work sent by mail, post-paid, on receipt 
of price, 


D. APPLETON & CO., Publishers, 


1, 3, & 5 Bonp Srreet, New York. 


Works Of Arabella B. Buckley. 


I 


Life and her Children. 


By AraBEeLLa B. Bucktey. With Illustrations. One vol., 12mo. Cloth. 
Price, $1.50. 


The work, with the author's usual felicity in captivating the youthful mind, 
discusses the structure and habits of the invertebrate animals. 


Ls 
Fairy-Land of Science. 


By Arase.ia B. Buckiey, author of “A Short History of Natural Sci- 
ence,” etc. With numerous Illustrations. 12mo. Cloth. Price, 
$1.50. 


‘* A child’s reading-book admirably adapted to the purpose intended. The 
young reader is referred to Nature itself rather than to books, and is taught to 
observe and investigate, and not to rest satisfied with a collection of dull defini 
tions learned by rote and worthless to the possessor. The present work will be 
found a valuable and interesting addition to the somewhat overcrowded child’s 
library.’’—Boston Gazette. 


oe ‘**Tt deserves to take a permanent place in the literature of youth.”—Zondon 
ames. 


‘**So interesting that, having once opened the book, we do not know how to 
leave off reading.’’—Saturday Review. 


III. 
A Short History of Natural Science and the 


Progress of Discovery, 


FROM THE TIME OF THE GREEKS TO THE PRESENT DAY. 
For Schools and Young Persons. By ArasBetta B. BUCKLEY. 
With Illustrations. 12mo. Cloth. Price, $2.00. 


‘¢ The volume is attractive as a book of anecdotes of men of science and their 
discoveries. Its remarkable features are the sound judgment with which the 
true landmarks of scientific history are selected, the conciseness of the informa- 
tion conveyed, and the interest with which the whole subject is nevertheless in- 
vested. Its style is strictly adapted to its avowed purpose of furnishing a text- 
book for the use of schools and young persons.’’—London Daily News. 


‘‘ A most admirable little volume. It is a classified réswmé of the chief discov- 
eries in physical science. To the young student it is a book to open up new 
worlds with every chapter.’’— Graphic. 

‘*The book will be a valuable aid in the study of the elements of natural 
science.”—Journal of Education. 


D. APPLETON & CO., Pusxiisners, 1, 8, & 5 Bonp Srrzet, N. Y. 


UNIVERSITY OF ILLINOIS-URBANA 


III 


3 0112 067859345 


